CHAPTER XVIII.

CLASSIFICATION AND ADDITIONAL STUDIES OF THE ALGÆ.

In order to show the general relationship of the algæ studied, the principal classes are here enumerated as well as some of the families. In some of the groups not represented by the examples studied above, a few species are described which may serve as the basis of additional studies if desired. The principal classes[17] of algæ are as follows:

Class Chlorophyceæ.

=331.= These are the green algæ, so called because the chlorophyll green is usually not masked by other pigments, though in some forms it is. There are three subclasses.

=332. Subclass PROTOCOCCOIDEÆ.=—In the Protococcoideæ are found the simplest green plants. Many of them consist of single cells which live an independent life. Others form “colonies,” loose aggregations of individuals not yet having attained the permanency of even a simple plant body, for the cells often separate readily and are able to form new colonies. The colonies are often held together by a gelatinous membrane, or matrix. Some are motile, while others are non-motile. A few of the families are here enumerated.

=333. Family Volvocaceæ.=—These are all motile, during the vegetative stage. The individuals are single or form more or less globose colonies.

=334. The “red snow” plant (Sphærella nivalis).=—This is often found in arctic and alpine regions forming a red covering over more or less large areas of snow or ice. For this reason it is called the “red snow plant.”

=335. Sphærella lacustris=, a closely related species, is very widely distributed in temperate regions along streams or on the borders of lakes and ponds. Here in dry weather it is often found closely adhering to the dry rock surface, and giving it a reddish color as if the rock were painted. This is especially the case in the shallow basins formed over the uneven surface of the rock near the water’s edge. These places during heavy rains or in high water are provided with sufficient water to fill the basins. During such times the red snow plant grows and multiplies, loses its red color and becomes green, and, being motile, is free swimming. It is a single-celled plant, oval in form, surrounded by a gelatinous sheath and with two cilia or flagella at the smaller end, by the vibration of which it moves (fig. 162). The single cell multiplies by dividing into two cells. When the water dries out of the basin, the motile plant comes to rest, and many of the cells assume the red color. To obtain the plant for study, scrape some of the red covering from these rock basins and place it in fresh spring water, and in a day or so the swarmers are likely to be found. Under certain conditions small microzoids are formed.

=336. Chlamydomonas= is a very interesting genus of motile one-celled green algæ, because the species are closely related to the Flagellates among the lower animals. The plant is oval, with a single chloroplast and surrounded by a gelatinous envelope through which the two cilia or flagella extend. One-celled organisms of this kind are sometimes called _monads_, i.e., a one-celled being. This one has a gelatinous cloak and is, therefore, a _cloaked monad_ (_Chlamydomonas_). The species often are found as a very thin green film on fresh water. C. pulvisculus is shown in fig. 163. When it multiplies the single cell divides into two, as shown in _B_. Sometimes a non-motile palmella stage is formed, as shown in _C_ and _D_. Reproduction takes place by gametes which are of unequal size, the smaller one representing the sperm and the larger one the egg, as in _E_ and _F_. These conjugate as in _G_ and _H_, the protoplasm of the smaller one passing over into the larger one, and a zygospore is thus formed.

=337. Of those which form colonies=, Pandorina morum is widely distributed and not rare. It consists of a sphere formed of sixteen individuals enclosed in a thin gelatinous membrane. Each cell possesses two cilia (or flagella), which extend from the broader end out through the enveloping membrane. By the movement of these flagella the colony goes rolling around in the water. When the plant multiplies each individual cell divides into sixteen small cells, which then grow and form new colonies. Reproduction takes place when the individual cells of the young colonies separate, and usually a small individual unites with a larger one and a zygospore is formed (see fig. 164). Eudorina elegans is somewhat similar, but when the gametes are formed certain mother cells divide into sixteen small motile males or sperms, and certain other mother cells divide into sixteen large motile females or eggs. These separate from the colonies, and the sperms pair with the eggs and fuse to form zygospores. This plant as well as Chlamydomonas pulvisculus foreshadows the early differentiation of sex in plants.

=338. Family Tetrasporaceæ.=—This family is well represented by Tetraspora lubrica forming slimy green net-like sheets attached to objects in slow-running water. It is really a single-celled plant. The rounded cells divide by cross walls into four cells, and these again, and so on, large numbers being held in loose sheets by the slime in which they are imbedded.

=339. Family Pleurococcaceæ.=—The members of this family are all non-motile in the vegetative stage. They consist of single individuals, or of colonies. Pleurococcus vulgaris (Protococcus vulgaris) is a single-celled alga, usually obtained with little difficulty. It is often found on the shaded, and cool, or moist side of trees, rocks, walls, etc., in damp places. This plant is not motile. It multiplies by fission (Fig. 165) into two, then four, etc. These cells remain united for a time, then separate. Sometimes the cells are found growing out into filaments, and it is thought by some that P. vulgaris may be only a simple stage of a higher alga. Eremosphæra viridis is another single-celled alga found in fresh water among filamentous forms. The cells are large and globose.

=340. Family Hydrodictyaceæ.=—These plants form colonies of cells. Hydrodictyon reticulatum, the water net, is made up of large numbers of cylindrical cells so joined at their ends as to form a large open mesh or net. Pediastrum forms circular flat colonies, as shown in fig. 166. Both of these plants are rather common in fresh-water pools, the latter one intermingled with filamentous algæ, while the former forms large sheets or nets. Multiplication in Hydrodictyon takes place by the protoplasm in one of the cells dividing into thousands of minute cells, which gradually arrange themselves in the form of a net, escape together from the mother cell, and grow into a large net. In Pediastrum multiplication takes place in a similar way, but the protoplasm in each cell usually divides into sixteen small cells, and escaping together from the mother cell arrange themselves and grow to full size (fig. 166).

=341. The Conjugateæ= include several families of green algæ, which probably should be included among the Chlorophyceæ. They have probably had their origin from some of the more simple members of the Protococcoideæ. They are represented by Spirogyra, Zygnema, and the desmids, studied in Chapter 14.

=342. Subclass CONFERVOIDEÆ.=—These are mostly filamentous algæ, the filaments being composed of cells firmly united, and, with the exception of the simplest forms, there is a definite growing point. A few of the families are as follows:

=343. Family Ulvaceæ.=—These contain the sea wracks, or sea lettuce, like Ulva, forming expanded green, ribbon-like growths in the sea.

=344. Family Ulotrichaceæ=, represented by Ulothrix zonata, not uncommon in slow-running water or in ponds of fresh water attached to rocks or wood. It consists of simple threads of short cells. Multiplication takes place by zoospores. Reproduction takes place by motile sexual cells (gametes) which fuse to form a zygospore (fig. 167).

=345. Family Chætophoraceæ=, represented by Chætophora (in Chapter 15) and Drapernaudia in fresh water.

=346. Family Œdogoniaceæ=, represented by Œdogonium (Chapter 16).

=347. Family Coleochætaceæ=, represented by Coleochæte (Chapter 17).

=348. Subclass SIPHONEÆ.=—There are several families.

=349. Family Botrydiaceæ.=—This is represented by Botrydium granulatum (Chapter 15, p. 146).

=350. Family Vaucheriaceæ=, represented by Vaucheria (Chapter 15), with quite a large number of species, is widely distributed.

Class Schizophyceæ (= Cyanophyceæ).

=351. The Blue-Green Algæ=, or =Cyanophyceæ= form slimy looking thin mats on damp wood or the ground, or floating mats or scum on the water. The color is usually bluish green, but in some species it is purple, red or brown. All have chlorophyll, but it is not in distinct chloroplasts and is more or less completely guised by the presence of other pigments. Two orders and eight families are recognized. The following include some of our common forms:

=352. ORDER COCCOGONALES (COCCOGONEÆ).=—Single-celled plants, occurring singly or in colonies, in some forms forming short threads. One of the two families is mentioned.

=353. Family Chroococcaceæ.=—The plants multiply only through cell division. Chroococcus, forms rounded, blue-green cells enclosed in a thick gelatinous coat, in fresh water and in damp places; certain species form “lichen-gonidia” in some genera of lichens. Glœocapsa is similar to Chroococcus, but the colonies are surrounded by an additional common gelatinous envelope (fig. 168); on damp rocks, etc.

=354. ORDER HORMOGONALES (HORMOGONEÆ).=—Plants filamentous, simple celled or with false or true branching, usually several celled (Spirulina is single celled). Multiplication takes place through _hormogones_, short sections of the threads becoming free; also through resting cells. Two of the six families are mentioned.

=355. Family Oscillatoriaceæ.=—This family is represented by the genus Oscillatoria, and by several other genera common and widely distributed. Oscillatoria contains many species. They are found on the damp ground or wood, or floating in mats in the water. They often form on the soil at the bottom of the pool, and as gas becomes entangled in the mat of threads, it is lifted from the bottom and floated to the surface of the water. The plant is thread-like, and divided up into many short cells. The threads often show an oscillating movement, whence the name _Oscillatoria_.

=356. Family Nostocaceæ.=—This family is represented by Nostoc, which forms rounded, slimy, blue-green masses on wet rocks. The individual plants in the slimy ball resemble strings of beads, each cell being rounded, and several of these arranged in chains as shown in fig. 170. Here and there are often found larger cells (heterocysts) in the chain. Nostoc punctiforme lives in the intercellular spaces of the roots of cycads (often found in greenhouses), and in the stems of Gunnera. N. sphæricum lives in the spaces between the cells in many species of liverworts (in the genera Anthoceros, Blasia, Pellia, Aneura, Riccia, etc.), and in the perforated cells of Sphagnum acutifolium. Anabæna is another common and widely distributed genus. The species occur in fresh or salt water, singly or in slimy masses. Anabæna azollæ lives endophytically in the leaves of the water fern, Azolla.

Class Schizomycetes.

=357. Bacteriales.=—The bacteria are sometimes classified with the Cyanophyceæ, under the name Schizophyta, and represent the subdivision Schizomycetes, or fission fungi, because many of them multiply by a division of the cells just as the blue-green algæ do. For example, Bacillus forms rods which increase in length and divide into two rods, or it may grow into a long thread of many short rods. Micrococcus consists of single rounded cells. Streptococcus forms chains of rounded cells, Sarcina forms irregular cubes of rounded cells, while others like Spirillum are spiral in form. Bacillus subtilis may be obtained by making an infusion from hay and allowing it to stand for several days. Bacillus tetani occurs in the soil, on old rusty nails, etc. It is called the tetanus bacillus because it causes a permanent spasm of certain muscles, as in “lockjaw.” This bacillus grows and produces this result on the muscles when it occurs in deep and closed wounds such as are caused by stepping on an old nail or other object which pierces the flesh deeply. In such a deep wound oxygen is deficient, and in this condition the bacillus is virulent. Opening the wounds to admit oxygen and washing them out with a solution of bichloride of mercury prevents the tetanus. Many bacteria are of great importance in bringing about the decay of dead animal and plant matter, returning it to a condition for plant food. (See also nitrate and nitrite bacteria, Chapter IX.) While most bacteria are harmless there are many which cause very serious diseases of man and animals, as typhoid fever, diphtheria, tuberculosis, etc., while some others produce disease in plants. Others aid in certain fermentations of liquids and are employed for making certain kinds of wines or other beverages. Some work in symbiosis with yeasts, as in the kephir yeast, used in fermenting certain crude beverages by natives of some countries.

=357=_a_. =Myxobacteriales (Myxobacteriaceæ Thaxter[18]).=—These plants consist of colonies of bacteria-like organisms, motile rods, which multiply by cross-division and secrete a gelatinous substance or matrix which surrounds the colonies. They form plasmodium-like masses which superficially resemble the slime moulds. In the fruiting stage some species become elevated from the substratum into cylindrical, clavate, or branched forms, which bear cysts of various shapes containing the rods in a resting stage, or the rods are converted into spore-like masses. Ex., Chondromyces crocatus on decaying plant parts, Myxobacter aureus on wet wood and bark, Myxococcus rubescens on dung, decaying lichens, paper, etc.

Class Flagellata.

=358. The flagellates= are organisms of very low organization resembling animals as much as they do plants. They are single celled and possess two cilia or flagella, by the vibration of which they move. Some are without a cell wall, while others have a well-defined membrane, but it rarely consists of cellulose. Some have chromatophores and are able to manufacture carbohydrates like ordinary green plants. These are green in Euglena, and brown in Hydrurus. Some possess a mouth-like opening and are able to ingest solid particles of food (more like animals), while others have no such opening and absorb food substances dissolved in water (more like plants). The Euglena viridis is not uncommon in stagnant water, often forming a greenish film on the water.

Class Peridineæ.

=358=_a_. These are peculiar one-celled organisms provided with two flagella and show some relationship to the Flagellates. They usually are provided with a cellulose membrane, which in some forms consists of curiously sculptured plates. In the higher forms this cellulose membrane consists of two valves fitting together in such a way as to resemble some of the diatoms. Like the Flagellates, some have green chromatophores, which in some are obscured by a yellow or brown pigment (resembling the diatoms), while still others have no chlorophyll. The Peridineæ are abundant in the sea, while some are found in fresh water.

Class Diatomaphyceæ (Bacillariales, Diatomaceæ).

=358=_b_. =The diatoms= are minute and peculiar organisms believed to be algæ. They live in fresh, brackish, and salt water. They are often found covering the surface of rocks, sticks, or the soil in thin sheets. They occur singly and free, or several individuals may be joined into long threads, or other species may be attached to objects by slender gelatinous stalks. Each protoplast is enclosed in a silicified skeleton in the form of a box with two halves, often shaped like an old-fashioned pill box, one half fitting over the other like the lid of a box. It is evident that in this condition the plant cannot increase much in size.

They multiply by fission. This takes place longitudinally, i.e., in the direction of the two halves or _valves_ of the box. Each new plant then has a valve only on one side. A new valve is now formed over the naked half, and fits inside the old valve. At each division the individuals thus become smaller and smaller until they reach a certain point, when the valves are cast off and the cell forms an _auxospore_, i.e., it grows alone, or after conjugation with another, to the full size again, and eventually provides itself with new valves. The valves are often marked, with numerous and fine lines, often making beautiful figures, and some are used for test objects for microscopes.

The free forms are capable of movement. The movement takes place in the longitudinal direction of the valves. They glide for some time in one direction, and then stop and move back again. It is not a difficult thing to mount them in fresh water and observe this movement.

The diatoms have small chlorophyll plates, but the green color is disguised by a brownish pigment called diatomin. The relationships of the diatoms are uncertain, but some, because of the color, think they are related to the Phæophyceæ.

Class Phæophyceæ.

=359. The brown algæ. (Phæophyceæ).=—The members of this class possess chlorophyll, but it is obscured by a brown pigment. The plants are accessible at the seashore, and for inland laboratories may be preserved in formalin (2½ per cent). (See also Chapter LVI.)

=360. Ectocarpus.=—The genus Ectocarpus represents well some of the simpler forms of the brown algæ (fig. 172). They are slender, filamentous branched algæ growing in tufts, either epiphytic on other marine algæ (often on Fucaceæ), or on stones. The slender threads are only divided crosswise, and thus consist of long series of short cells. The sporangia are usually plurilocular (sometimes unilocular), and usually occur in the place of lateral branches. The zoospores escape from the apex of the sporangium and are biciliate, and they fuse to form zygospores.

=361. Sphacelaria.=—The species of this genus represent an advance in the development of the thallus. While they are filamentous and branched, division takes place longitudinally as well as crosswise (fig. 173).

=362. Leathesia difformis= represents an interesting type because the plant body is small, globose, later irregular and hollow, and consists of short radiately arranged branches, the surface ones in the form of short, crowded, but free, trichome-like green branches. This trichothallic body recalls the similar form of Chætophora pisiformis (Chapter 16) among the Chlorophyceæ.

=363. The Giant Kelps.=—Among the brown algæ are found the largest specimens, some of the laminarias or giant kelps, rivaling in size the largest land plants, and some of them have highly developed tissues. _Postelsia palmæformis_ has a long, stout stem, from the free end of which extend numerous large and long blades, while the stem is attached to the rocks by numerous “root” like outgrowths, the holdfasts. It occurs along the northern Pacific coast, and appears to flourish where it receives the shock of the surf beating on the shore. Several species of Laminaria occur on our north Atlantic coast. In L. digitata, the stem expands at the end into a broad blade, which becomes split into several smaller blades (fig. 174). _Macrocystis pyrifera_ inhabits the ocean in the southern hemisphere, and sometimes is found along the north American coast. It is said to reach a length of 200-300 meters.

=364. Fucus, or Rockweed.=—This plant is a more or less branched and flattened thallus or “frond.” One of them, illustrated in fig. 119, measures 15-30 _cm_ (6-12 inches) in length. It is attached to rocks and stones which are more or less exposed at low tide. From the base of the plant are developed several short and more or less branched expansions called “holdfasts,” which, as their name implies, are organs of attachment. Some species (F. vesiculosus) have vesicular swellings in the thallus.

The fruiting portions are somewhat thickened as shown in the figure. Within these portions are numerous oval cavities opening by a circular pore, which gives a punctate appearance to these fruiting cushions. Tufts of hairs frequently project through them.

=365. Structure of the conceptacles.=—On making sections of the fruiting portions one finds the walls of the cavities covered with outgrowths. Some of these are short branches which bear a large rounded terminal sac, the oogonium, at maturity containing eight egg-cells. More slender and much-branched threads bear narrowly oval antheridia. In these are developed several two-ciliated spermatozoids.

=366. Fertilization.=—At maturity the spermatozoids and egg-cells float outside of the oval cavities, where fertilization takes place. The spermatozoid sinks into the protoplasm of the egg-cell, makes its way to the nucleus of the egg, and fuses with it as shown in fig. 181. The fertilized egg then grows into a new plant. Nearly all the brown algæ are marine.

=367. The Gulf weed= (=Sargassum bacciferum=) in the warmer Atlantic ocean unites in great masses which float on the water, whence comes the name “Sargassum Sea.” The Sargassum grows on the coast where it is attached to the rocks, but the beating of the waves breaks many specimens loose and these float out into the more quiet waters, where they continue to grow and multiply vegetatively.

=368. Uses.=—Laminaria japonica and L. angustata are used as food by the Chinese and Japanese. Some species of the Laminariaceæ are used as food for cattle and are also used for fertilizers, while L. digitata is sometimes employed in surgery.

_Classification._—Kjellman divides the Phæophyceæ into two orders.

=369. Order Phæosporales (Phæosporeæ)= including 18 families. One of the most conspicuous families is the Laminariaceæ, including among others the Giant Kelps mentioned above (Laminaria, Postelsia, Macrocystis, etc.).

=370. Order Cyclosporales (Cyclosporeæ).=—This includes one family, the _Fucaceæ_ with Ectocarpus, Sphacelaria, Læathesia, Fucus, Sargassum, etc.

Class Rhodophyceæ.

=371. The red algæ (Rhodophyceæ).=—The larger number of the so-called red algæ occur in salt water, though a few genera occur in fresh water. The plants possess chlorophyll, but it is usually obscured by a reddish or purple pigment.

=372. Nemalion.=—This is one of the lower marine forms, though its thallus is not one of the simplest in structure. The plant body consists of a slender cylindrical branched shoot, sometimes very profusely branched. The central strand is rather firm, while the cortex is composed of rather loose filaments.

=373. Batrachospermum.=—This genus occurs in fresh water, and the species are found in slow-running water of shallow streams or ditches. There is a central slender strand which is more or less branched, and on these branches are whorls of densely crowded slender branches occurring at regular intervals. The plants are usually very slippery. Gonidia are formed on the ends of some of these branches in globose sporangia, called monosporangia, since but a single spore or gonidium is developed in each. Other branches often terminate in long slender hyaline setæ.

=374. Lemanea.=—This genus also occurs in fresh water. The species develop only during the cold winter months in rapids of streams or where the water from falls strikes the rocks and is thoroughly aerated. They form tufts of greenish threads, cylindrical or whiplike, which in the summer are usually much broken down. The threads are hollow and have a firm cortex. These are the sexual shoots, and they arise as branches from a sterile filamentous-branched, Chantransia-like form.

=375. Fertilization in the lower red algæ.=—The sexual organs in the red algæ consist of antheridia and carpogonia. The antheridia are usually borne in crowded clusters, or surfaces, and bear terminally the small non-motile sperm cells. The carpogonium is a branch of one or several cells, the terminal cell (procarp) extending into a long slender process, the trichogyne. The sperm cell comes in contact with the trichogyne, and in the case of Nemalion and some others the nucleus has been found to pass down the inside and fuse with the nucleus of the procarp.

From this point in the lower red algæ like Nemalion, Batrachospermum and Lemanea the formation of the spores is very simple. The procarp is stimulated to growth, and buds in different directions, producing branched chains of spores (carpospores). The carpospores form a rather compact cluster called the sporocarp, which means spore-fruit or spore-fruit body. In Batrachospermum it is seen as a compact tuft in the loose branching, in Nemalion it lies in the surface of the cortex, while in Lemanea the sporocarps lie at different positions in the hollow tube of the sexual shoot.

=376. Gonidia in the red algæ.=—The common type of gonidium in the red algæ is found in the _tetraspores_. A single mother cell divides into four cells arranged usually in the form of tetrads within the _tetrasporangium_. In Callithamnion the tetrasporangium is exposed. In Polysiphonia, Rhabdonia, Gracilaria, etc., it is imbedded in the cortex. In Batrachospermum there are monosporangia, each monosporangium containing a single gonidium, while in Lemanea, and according to some also in Nemalion, gonidia are wanting.

=377. Gracilaria.=—Gracilaria is one of the marine forms, and one species is illustrated in fig. 185. It measures 15-20_cm_ or more long, and is profusely branched in a palmate manner. The parts of the thallus are more or less flattened. The fruit is a cystocarp, which is characteristic of the Rhodophyceæ (Florideæ). In Gracilaria these fruit bodies occur scattered over the thallus. They are somewhat flask-shaped, are partly sunk in the thallus, and the conical end projects strongly above the surface. The carpospores are grouped in radiating threads within the oval cavity of the cystocarp. These cystocarps are developed as a result of fertilization. Other plants bear gonidia in groups of four, the so-called _tetraspores_.

=378. Rhabdonia.=—This plant is about the same size as the gracilaria, though it possesses more filiform branches. The cystocarps form prominent elevations, while the carpospores lie in separated groups around the periphery of a sterile tissue within the cavity. (See figs. 187, 188.) Gonidia in the form of tetraspores are also developed in Rhabdonia.

=379. Fertilization of the higher red algæ.=—The process of fertilization in most of the red algæ is very complicated, chiefly because the fertilized egg-cell (procarp) does not develop the spores directly, as in Nemalion, Lemanea, etc., but fuses directly, or by a short cell or long filament with one or more auxiliary cells before the sporocarp is finally formed. Examples are Rhabdonia, Polysiphonia, Callithamnion, Dudresnaya, etc. (fig. 189). The auxiliary cell then develops the sporocarp. See fig. 189 for conjugation of a filament from the fertilized procarp with an auxiliary cell.

=380. Uses of the red algæ.=—Many species produce a great amount of gelatinous substance in their tissues, and several of these are used for food, for the manufacture of gelatines and agar-agar. Some of these are Gracilaria lichenoides and wrightii, the former species occurring along the coast of India and China. The plant is easily converted into gelatinous substance (agar-agar). Chondrus crispus, widely distributed in the northern Atlantic is known as “Irish” moss and is used for food and for certain medicinal purposes. Gigartina mamillosa in the Atlantic and Arctic oceans is similarly employed. The following orders are recognized in the red algæ:

=381. Order Bangiales.=—Example, Bangia atropurpurea (= Conferva atropurpurea) in springs and brooks in North America and Europe. Porphyra contains a number of species forming broad, thin, leaf-like purple sheets in the sea.

=382. Order Nemalionales.=—Including Lemanea, Batrachospermum, Nemalion, described above, and many others.

=383. Order Gigartinales.=—In this order occurs the common Iceland moss (Chondrus crispus) in the sea, and Rhabdonia and Gigartina mentioned above.

=384. Order Rhodomeniales.=—In this order occurs Gracilaria and Polysiphonia mentioned above, also the beautiful marine forms like Ceramium.

=385. Order Cryptonemiales.=—Examples are Dudresnaya, Melobesia, Corallina, etc., the last two genera include many species with a wide distribution.

Class Charophyceæ, Order Charales.

=386.= The Charales are by some thought to represent a distinct class of algæ standing near the mosses, perhaps, because of the biciliate character of the spermatozoids. There is one family, the Characeæ. The plants occur in fresh and brackish water. Aside from the peculiarity of the reproductive organs they are remarkable for the large size of the cells of the internodes and of the “leaves,” and the protoplasm exhibits to a remarkable degree the phenomenon of “cyclosis” (see paragraphs 17-20). Three of the genera are found in North America (Chara, Nitella (Fig. 8) and Tolypella).

=386a.= The complicated structure of the sexual organs shows a higher state of organization than any of the other living algæ known. While the internodes in Nitella are composed of a single, stout cell, some times a foot or more in length, the nodes in all are composed of a group of smaller cells. From the lateral cells of this group lateral axes (sometimes called leaves) arise in whorls.

In Nitella the internodes are naked, but in most species of Chara they are _corticated_, i.e., they are covered by a layer of numerous elongated cells which grow downward from the nodes at the base of the whorl of lateral shoots.

=386b.= The sexual organs are situated at the nodes of the whorled lateral shoots, and consist of antheridia and carpogonia. Most of the plants are monœcious, and both antheridia and carpogonia are often attached to the same node, the antheridium projecting downward while the carpogonium is more or less ascending. The sexual organs are visible to the unaided eye. The antheridium is a globose red body of an exceedingly complicated structure. The sperms are borne in several very long coiled slender threads which are divided transversely into numerous cells. The carpogonium is oval or elliptical in outline, the wall of which is composed of several closely coiled spiral threads enclosing the large egg.

FOOTNOTES:

[17] In Engler & Prantl’s Pflanzenfamilien, Wille uses the term class for these principal subdivisions of the algæ. Systematists are not yet agreed upon a uniform use of the terms.

[18] See Bot. Gaz., 17, 389, 1892.