CHAPTER XXXIV.

FURTHER STUDIES ON GYMNOSPERMS.

Cycas.

=627.= In such gymnosperms as cycas, illustrated in the frontispiece, there is a close resemblance to the members of the fern group, especially the ferns themselves. This is at once suggested by the form of the leaves. The stem is short and thick. The leaves have a stout midrib and numerous narrow pinnæ. In the center of this rosette of leaves are numerous smaller leaves, closely overlapping like bud scales. If we remove one of these at the time the fruit is forming we see that in general it conforms to the plan of the large leaves. There are a midrib and a number of narrow pinnæ near the free end, the entire leaf being covered with woolly hairs. But at the lower end, in place of the pinnæ, we see oval bodies. These are the macrosporangia (ovules) of cycas, and correspond to the macrosporangia of selaginella, and the leaf is the macrosporophyll.

=628. Female prothallium of cycas.=—In figs. 367, 368, are shown mature ovules, or macrosporangia, of cycas. In 368, which is a roentgen-ray photograph of 367, the oval prothallium can be seen. So in cycas, as in selaginella, the female prothallium is developed entirely inside of the macrosporangium, and derives the nutriment for its growth from the cycas plant, which is the sporophyte. Archegonia are developed in this internal mass of cells. This aids us in determining that it is the prothallium. In cycas it is also called endosperm, just as in the pines.

=629.= If we cut open one of the mature ovules, we can see the endosperm (prothallium) as a whitish mass of tissue. Immediately surrounding it at maturity is a thin, papery tissue, the remains of the nucellus (macrosporangium), and outside of this are the coats of the ovule, an outer fleshy one and an inner stony one.

=630. Microspores, or pollen, of cycas.=—The cycas plant illustrated in the frontispiece is a female plant. Male plants also exist which have small leaves in the center that bear only microsporangia. These leaves, while they resemble the ordinary leaves, are smaller and correspond to the stamens. Upon the under side, as shown in fig. 369, the microsporangia are borne in groups of three or four, and these contain the microspores, or pollen grains. The arrangement of these microsporangia on the under side of the cycas leaves bears a strong resemblance to the arrangement of the sporangia on the under side of the leaves of some ferns.

=631. The gingko tree= is another very interesting plant belonging to this same group. It is a relic of a genus which flourished in the remote past, and it is interesting also because of the resemblance of the leaves to some of the ferns like adiantum, which suggests that this form of the leaf in gingko has been inherited from some fern-like ancestor.

=632.= While the resemblance of the leaves of some of the gymnosperms to those of the ferns suggests fern-like ancestors for the members of this group, there is stronger evidence of such ancestry in the fact that a prothallium can well be determined in the ovules. The endosperm with its well-formed archegonia is to be considered a prothallium.

=633. Spermatozoids in some gymnosperms.=—But within the past two years it has been discovered in gingko, cycas, and zamia, all belonging to this group, that the sperm cells are well-formed spermatozoids. In zamia each one is shaped somewhat like the half of a biconvex lens, and around the convex surface are several coils of cilia. After the pollen tube has grown down through the nucellus, and has reached a depression at the end of the prothallium (endosperm) where the archegonia are formed, the spermatozoids are set free from the pollen tube, swim around in a liquid in this depression, and later fuse with the egg. In gingko and cycas these spermatozoids were first discovered by Ikeno and Hirase in Japan, and later in zamia by Webber in this country. In figs. 371-374 the details of the male prothallia and of fertilization are shown.

=634. The sporophyte in the gymnosperms.=—In the pollen grains of the gymnosperms we easily recognize the characters belonging to the spores in the ferns and their allies, as well as in the liverworts and mosses. They belong to the same series of organs, are borne on the same phase or generation of the plant, and are practically formed in the same general way, the variations between the different groups not being greater than those within a single group. These spores we have recognized as being the product of the sporophyte. We are able then to identify the sporophyte as that phase or generation of the plant formed from the fertilized egg and bearing ultimately the spores. We see from this that the sporophyte in the gymnosperms is the prominent part of the plant, just as we found it to be in the ferns. The pine tree, then, as well as the gingko, cycas, yew, hemlock-spruce, black spruce, the giant redwood of California, etc., are sporophytes.

While the sporangia (anther sacs) of the male flowers open and permit the spores (pollen) to be scattered, the sporangia of the female flowers of the gymnosperms rarely open. The macrospore is developed within sporangium (nucellus) to form the female prothallium (endosperm).

=635. The gametophyte has become dependent on the sporophyte.=—In this respect the gymnosperms differ widely from the pteridophytes, though we see suggestions of this condition of things in Isoetes and Selaginella, where the female prothallium is developed within the macrospore, and even in Selaginella begins, and nearly completes, its development while still in the sporangium.

In comparing the female prothallium of the gymnosperms with that of the fern group we see a remarkable change has taken place. The female prothallium of the gymnosperms is very much reduced in size. Especially, it no longer leads an independent existence from the sporophyte, as is the case with nearly all the fern group. It remains enclosed within the macrosporangium (in cycas if not fertilized it sometimes grows outside of the macrosporangium and becomes green), and derives its nourishment through it from the sporophyte, to which the latter remains organically connected. This condition of the female prothallium of the gymnosperms necessitated a special adaptation of the male prothallium in order that the sperm cells may reach and fertilize the egg-cell.

=636. Gymnosperms are naked seed plants.=—The pine, as we have seen, has naked seeds. That is, the seeds are not enclosed within the carpel, but are exposed on the outer surface. All the plants of the great group to which the pine belongs have naked seeds. For this reason the name “_gymnosperms_” has been given to this great group.

=637. Classification of gymnosperms.=—The gingko tree has until recently been placed with the pines, yew, etc., in the order _Pinales_, but the discovery of the spermatozoids in the pollen tube suggests that it is not closely allied with the Pinales, and that it represents an order coordinate with them. Engler arranges the living gymnosperms somewhat as follows:

Class Gymnospermæ.

Order 1. Cycadales; family Cycadaceæ. Cycas, Zamia, etc. Order 2. Gingkoales; family Gingkoaceæ. Gingko. Order 3. Pinales (or Coniferæ); family 1. Taxaceæ. Taxus, the common yew in the eastern United States, and Torreya, in the western United States, are examples. family 2. Pinaceæ. Sequoia (redwood of California), firs, spruces, pines, cedars, cypress, etc. Order 4. Gnetales. Welwitschia mirabilis, deserts of southwest Africa; Ephedra, deserts of the Mediterranean and of West Asia. Gnetum, climbers (Lianas), from tropical Asia and America.

=638.= TABLE SHOWING HOMOLOGIES OF SPOROPHYTE AND GAMETOPHYTE IN THE PINE.

TERMS CORRESPONDING TO THOSE USED IN PTERIDOPHYTES. COMMON TERMS.

-------------+-------------------------------------------------- | Sporophyte = Pine tree. | Spore-bearing part = Male and female cones. -------------+-------------------------------------------------- Sporophyte | Microsporophyll = Stamen. | Microsporangium = Pollen sac. -------------+-------------------------------------------------- | Microspore = Pollen grain. | Mature microspore is = Mature pollen grain. | rudimentary male | prothallium with | rudimentary | antheridium | Large cell (part of = Vegetative cell | antheridium wall?) of pollen grain. | Antheridium cell = Small cell of pollen Male | grain. gametophyte | Antheridium cell divides = Generative cell. | to form stalk cell and | central cell of | antheridium | (male sexual organ) | Central cell of = Paternal cells, | antheridium or generative cells. | divides to form | two sperm cells -------------+-------------------------------------------------- | Macrosporophyll = Ovuliferous scale (cover | scale and carpellary | outgrowth); or three | carpels united into Sporophyte | ovuliferous scale, | the central one sterile | (in axil of cover scale). | Macrosporangium covered = Nucellus covered by | by integument integument = ovule. -------------+-------------------------------------------------- | Macrospore (remains in = Large cell in center of | sporangium) nucellus which develops | embryo sac and endosperm | (remains in nucellus). Female | Female prothallium = Endosperm, in nucellus. gametophyte | (in sporangium) | Archegonia (female = Corpuscula, in endosperm. | sexual organs) | Egg = Maternal cell, or germ | cell. -------------+-------------------------------------------------- | Egg (fertilized) = Germ cell. | Young sporophyte = Pine embryo in nucellus | and integument. Young | Young sporophyte = Embryo | sporophyte | In remains of = Endosperm | | gametophyte = Nucellus | Seed. | And sporangium = Integument | | Surrounded by new | | growth of old | | sporophyte | -------------+---------------------------------------+----------