CHAPTER XXIV.

LIVERWORTS CONTINUED.

=488. Sporogonium of marchantia.=—If we examine the plant shown in fig. 181 we shall see oval bodies which stand out between the rays of the female receptacle, supported on short stalks. These are the sporogonia, or spore-cases. We judge at once that they are quite different from those which we have studied in riccia, since those were not stalked. We can see that some of the spore-cases have opened, the wall splitting down from the apex in several lines. This is caused by the drying of the wall. These tooth-like divisions of the wall now curl backward, and we can see the yellowish mass of the spores in slow motion, falling here and there. It appears also as if there were twisting threads which aided the spores in becoming freed from the capsule.

=489. Spores and elaters.=—If we take a bit of this mass of spores and mount it in water for examination with the microscope, we shall see that, besides the spores, there are very peculiar thread-like bodies, the markings of which remind one of a twisted rope. These are very long cells from the inner part of the spore-case, and their walls are marked by spiral thickenings. This causes them in drying, and also when they absorb moisture, to twist and curl in all sorts of ways. They thus aid in pushing the spores out of the capsule as it is drying.

=490. Sporophyte of marchantia compared with riccia.=—We must recollect that the sporogonium in marchantia is larger than in riccia, and that it is also not lying in the tissue of the thallus, but is only attached to it at one side by a slender stalk. This shows us an increase in the size and complex structure of this new phase of the plant, the _sporophyte_. This is one of the very interesting things which we have to note as we go on in the study of the higher plants.

=491. Sporophyte dependent on the gametophyte for its nutriment.=—We thus see that at no time during the development of the sporogonium is it independent from the gametophyte. This new phase of plants then, the sporophyte, has not yet become an independent plant, but must rely on the earlier phase for sustenance.

=492. Development of the sporogonium.=—It will be interesting to note briefly how the development of the marchantia sporogonium differs from that of riccia. The first division of the fertilized egg is the same as in riccia, that is a wall which runs crosswise of the axis of the archegonium divides it into two cells. In marchantia the cell at the base develops the stalk, so that here there is a radical difference. The outer cell forms the capsule. But here after the wall is formed the inner tissue does not all go to make spores, as is the case with riccia. But some of it forms the elaters. While in riccia only the outside layer of cells of the sporogonium remained sterile, in marchantia the basal half of the egg remains completely sterile and develops the stalk, and in the outer half the part which is formed from some of the inner tissue is also sterile.

=493. Embryo.=—In the development of the embryo we can see all the way through this division line between the basal half, which is completely sterile, and the outer half, which is the fertile part. In fig. 267 we see a young embryo, and it is nearly circular in section although it is composed of numerous cells. The basal half is attached to the base of the inner surface of the archegonium, and at this time the archegonium still surrounds it. The archegonium continues to grow then as the embryo grows, and we can see the remains of the shrivelled neck. The portion of the embryo attached to the base of the archegonium is the sterile part and is called the “foot,” and later develops the stalk. The sporogonium during all the stages of its development derives its nourishment from the gametophyte at this point of attachment at the base of the archegonium. Soon, as shown in fig. 267 at the right, the outer portion of the sporogonium begins to differentiate into the cells which form the elaters and those which form spores. These lie in radiating lines side by side, and form what is termed the _archesporium_. Each fertile cell forms four spores just as in riccia. They are thus called the mother cells of the spores, or spore mother cells.

=494. How marchantia multiplies.=—New plants of marchantia are formed by the germination of the spores, and growth of the same to the thallus. The plants may also be multiplied by parts of the old ones breaking away by the action of strong currents of water, and when they lodge in suitable places grow into well-formed plants. As the thallus lives from year to year and continues to grow and branch the older portions die off, and thus separate plants may be formed from a former single one.

=495. Buds, or gemmæ, of marchantia.=—But there is another way in which marchantia multiplies itself. If we examine the upper surface of such a plant as that shown in fig. 268, we shall see that there are minute cup-shaped or saucer-shaped vessels, and within them minute green bodies. If we examine a few of these minute bodies with the microscope we see that they are flattened, biconvex, and at two opposite points on the margin there is an indentation similar to that which appears at the growing end of the old marchantia thallus. These are the growing points of these little buds. When they free themselves from the cups they come to lie on one side. It does not matter on what side they lie, for whichever side it is, that will develop into the lower side of the thallus, and forms rhizoids, while the upper surface will develop the stomates.

Leafy-stemmed liverworts.

=496.= We should now examine more carefully than we have done formerly a few of the leafy-stemmed liverworts (called foliose liverworts).

=497. Frullania= (Fig. 32).—This plant grows on the bark of logs, as well as on the bark of standing trees. It lives in quite dry situations. If we examine the leaves we will see how it is able to do this. We note that there are two rows of lateral leaves, which are very close together, so close in fact that they overlap like the shingles on a roof. Then, as the creeping stems lie very close to the bark of the tree, these overlapping leaves, which also hug close to the stem and bark, serve to retain moisture which trickles down the bark during rains. If we examine these leaves from the under side as shown in fig. 34, we see that the lower or basal part of each one is produced into a peculiar lobe which is more or less cup-shaped. This catches water and holds it during dry weather, and it also holds moisture which the plant absorbs during the night and in damp days. There is so much moisture in these little pockets of the under side of the leaf that minute animals have found them good places to live in, and one frequently discovers them in this retreat. There is here also a third row of poorly developed leaves on the under side of the stem.

=498. Porella.=—Growing in similar situations is the plant known as porella. Sometimes there are a few plants in a group, and at other times large mats occur on the bark of a trunk. This plant, porella, also has closely overlapping leaves in rows on opposite sides of the stem, and the lower margin of each leaf is curved under somewhat as in frullania, though the pocket is not so well formed.

The larger plants are female, that is they bear archegonia, while the male plants, those which bear antheridia, are smaller and the antheridia are borne on small lateral branches. The antheridia are borne in the axils of the leaves. Others of the leafy-stemmed liverworts live in damp situations. Some of these, as Cephalozia, grow on damp rotten logs. Cephalozia is much more delicate, and the leaves are farther apart. It could not live in such dry situations where the frullania is sometimes found. If possible the two plants should be compared in order to see the adaptation in the structure and form to their environment.

=499. Sporogonium of a foliose liverwort.=—The sporogonium of the leafy-stemmed liverworts is well represented by that of several genera. We may take for this study the one illustrated in fig. 274, but another will serve the purpose just as well. We note here that it consists of a rounded capsule borne aloft on a long stalk, the stalk being much longer proportionately than in marchantia. At maturity the capsule splits down into four quadrants, the wall forming four valves, which spread apart from the unequal drying of the cells, so that the spores are set free, as shown in fig. 276. Some of the cells inside of the capsule develop elaters here also as well as spores. These are illustrated in fig. 278.

=500.= In this plant we see that the sporophyte remains attached to the gametophyte, and thus is dependent on it for sustenance. This is true of all the plants of this group. The sporophyte never becomes capable of an independent existence, and yet we see that it is becoming larger and more highly differentiated than in the simple riccia.

The Horned Liverworts.[25]

=501. The horned liverworts= take their name from the shape of the sporogonium. This is long, slender, cylindrical, pointed, and very slightly curved, suggesting the shape of a minute horn. Anthoceros is one of the most common and widely distributed species. The plant grows on damp soil or on mud.

Anthoceros.

=502. The gametophyte.=—The gametophyte is thalloid. It is thin, flattened, green, irregularly ribbon-shaped and branched. It lies on the soil and is more or less crisped or wavy, or curled, the edges nearly plane, or somewhat irregular, and with minute lobes, or notches, especially near the growing end. The general form and branching can be seen in fig. 279. Where the plants are much crowded the thallus is more irregular, and often possesses numerous small lateral branches in addition to the main lobes. Upon the under side are the slender rhizoids, which attach to the soil. With a hand lens there can be seen also upon the under side small dark, rounded and thickened spots, where an alga (nostoc) is located.

Sexual Organs of Anthoceros.

=502. The sexual organs of anthoceros= differ considerably from those of the other liverworts studied. In the first place they are immersed in the true tissue of the thallus, i.e., they do not project above the surface.

=503. Antheridia.=—The antheridium arises from an internal cell of the thallus, a cell just below the upper surface. This cell develops usually a group of antheridia which lie in a cavity formed around this cell as the thallus continues to grow. They are situated along the middle line of the thallus, and can be seen by making a section in this direction. The antheridia are oval or rounded, have a wall of one layer of cells which contains the sperm cells, and each antheridium has a slender stalk. The sperms are like those of the true liverworts.

=504. Archegonia.=—The archegonia are also borne along the middle line of the thallus. Each one arises at an early stage in the development of the tissue of the thallus from a superficial cell, but the archegonium does not project above the surface. The venter therefore which contains the egg is deep down in the thallus, the wall of the neck is formed from cells indistinguishable from the adjoining cells of the thallus and opens at the surface.

Sporophyte of Anthoceros.

=505. The Sporogonium.=—The sporogonium is developed from the fertilized egg, fertilization resulting of course from the fusion of one of the sperms with the nucleus of the egg. From the lower part of the embryo certain cells elongate and push out like rhizoids into the thallus (gametophyte), but never reach the outside so that the sporogonium derives its nutriment from the gametophyte in a parasitic manner like the true liverworts. It is surrounded at the base by a sheath, an outgrowth of the gametophyte.

=506. Growing point of the sporogonium.=—A remarkable thing about the sporogonium of anthoceros, and its relatives, is that the growing point instead of being situated at the free end is located near the base, just above the nourishing foot. Thus the upper part of the sporogonium is older. In the old sporogonia there may be ripe spores near the free end, young ones near the middle, and undifferentiated growing tissue near the base. A longitudinal section of a sporogonium just as the spores are ripening will show this.

=507. Structure of the sporogonium.=—A longitudinal section of the sporogonium shows that the spore-bearing tissue occupies a comparatively small portion of the sporogonium. In the section there is a narrow layer (two cells thick) on either side and joined at the top. In the entire sporogonium this fertile tissue is in the shape of an inverted test tube situated inside of the sporogonium. The wall of the sporogonium is about four cells thick. The sterile tissue inside of the spore-bearing tube is the columella. The cells of the wall contain chlorophyll, and there are true stomata with guard cells in the epidermal layer.

=508. Spores and elaters.=—In the spore-bearing tissue there are two layers of cells (the archesporium). Each cell is a potential mother cell. The cells, however, of alternate tiers do not form spores. They elongate some what and are somewhat irregular and sometimes divide or branch. They are supposed to represent rudimentary _elaters_. The cells in the other tiers are actual mother cells, and each one forms four spores.

=509. The sporophyte of anthoceros= represents the highest type found in the liverworts. The spongy green parenchyma forming the wall, with the stomata in the epidermal layer, fits this tissue for the process of photosynthesis, so that this part of the sporophyte functions as the green leaf of the seed plants. It has been suggested by some that if the rhizoids on the nourishing foot could only extend outside and anchor in the soil, the sporophyte of anthoceros could live an independent existence. But we see that it stops short of that.

Classification of the Liverworts.

CLASS HEPATICÆ.

=510. Order Marchantiales.=[26]—There are two families represented in the United States.

Family Ricciaceæ, including Riccia and Ricciocarpus.

Family Marchantiaceæ, including Marchantia, Fegatella (= Conocephalus), Fimbriaria, Targionia, etc.

=511. Order Jungermanniales.=[27]—There are two subdivisions of this order. _The Anacrogynæ_ include chiefly thalloid forms with continued apical growth, the archegonia back of the apical cell. Examples: Blasia, Aneura, Pellia, etc.

_The Acrogynæ_ include chiefly foliose forms, the archegonia arising from the apical cell and in such cases interrupting apical growth. Examples: Cephalozia, Frullania, Bazzania, Jungermannia, Ptilidium, Porella, etc.

CLASS ANTHOCEROTES.

=512. The Anthocerotes= have formerly been placed with the Hepaticæ as an order. But because of their wide divergence from the other liverworts in the development of the sexual organs, and especially in the structure of the sporophyte, they are now by some separated as a distinct class. There is one order.

=Order Anthocerotales.=[28]—This includes one family (Anthocerotaceæ) with Anthoceros and Notothylas in Europe and North America, and Dendroceros in the tropics. The latter is epiphytic.

FOOTNOTES:

[25] May be used as an alternate study for marchantia.

[26] As subclass in Engler and Prantl.

[27] As subclass in Engler and Prantl.

[28] As subclass in Engler and Prantl.