260 



KNOWLEDGE. 



[No^^MBEB 1, 1898. 



under sides of the fronds. If the spores are allowed to 

 germinate under proper conditions they grow into little 

 green cellular structures, the so-calledP/o?/(rt//(', which lead 

 a short but independent existence, form true Archegonia 

 and Antheridia, and constitute, therefore, the real Oophyte, 

 or egg-bearing stage in the life-cycle. In other words, the 

 Ociphyte is a reduced, simplified, and transitory stage in 

 comparison with the highly-specialized, long-lived Sporo- 

 phyte which we see at its highest in the tree-ferns of the 

 Tropics. 



If we look round for other types to help us in tracing 

 the course of development of higher plant-life, we naturally 

 turn first to those relatives of the ferns which are com- 

 monly known as the "horse-tails," "club-mosses," "quill- 

 worts," and " pill-worts." Of all these, there is no doubt 

 that we can most easily obtain the information we want 

 from observation of the genus Selni/inella. 



The species included under this name are for the most 

 part spreading, low-growing plants with creeping stems, 

 sending off more or less upright shoots. The structure of 

 their tissues is in general like that of ferns : they have a 

 well developed fibro-vascular system; leaves (usually of 

 two kinds) of several cell-layers in thickness and with 

 distinct midribs ; and roots developed on slender cylindrical 

 outgrowths of the stem known as )/ii:(iplinres. 



In our own country we have only one species, S. selagi- 

 noiiles, which is not uncommon in the moist and rocky 

 parts of our mountain districts. So many foreign species 

 are, however, now in cultivation and easily obtainable, 

 that there can be no trouble in getting material for study. 

 The specific differences are mainly those of leaf arrange- 

 ment and internal stem structure, and will raise no difficulty 

 in our line of observation. Several of the illustrations here 

 used are of Sdai/ineUn s/iinulofia, A. Br., a common species 

 in the Swiss mountains, not unlike our native form in 

 size and general habit. 



If we commence, as in the case of the fern, by looking 

 at the mature plant (Figs. I and K), with a view to finding 

 the nature of its reproductive organs, we find that toward 

 the tips of the erect shoot the leaves are closely crowded 

 together, forming spikes or cones. 



These leaves do not show any brown groups of spore- 

 cases on their under side like the fern-fronds, but if they 

 are stripped off, or if a section is cut along the axis of the 

 spike, it will be found that they differ from the ordinary 

 leaves in bearing a very distinct sporangium at the inner 

 and upper aspect of the leaf base. Closer observation 

 shows that these sporangia are distinctly of two kinds. 

 Those nearer the tip are transversely oval, single- 

 chambered sacs ; green at first, then yellow, splitting 

 across when ripe and discharging a yellow dust (Fig. M). 

 Those nearer the base of the cone are larger, and each 

 consists of four rounded or slightly angular lobes, one 

 resting on the other three in the manner in which the 

 round cannon balls of olden artillery were piled ; or, to put 

 it geometrically, the centre of each corresponding in posi- 

 tion to one of the solid angles of a tetrahedron.* 



Here, then, we meet with a very distinct difference 

 between this plant and the fern. There are two kinds of 

 sporangia instead of one. The next step is to look at the 

 contents of these two sporangia. 



At the base of the cone, if it is fairly ripe, some of the 



* This tetrabedral arrangement results from a differenre iu the 

 divieion-planes of the cell from which the epore-group is formed. 

 Thus, if a spherical cell ie divided by a transverse wall, and the two 

 segments become rounded off, a pair of cells like a figure eight is 

 produced. If now each of tliese becomes similarly divided, but one 

 in a horizontal plane and the other in a vertical one, it is evident that 

 the resulting cells will naturally aci^uire this peculiar grouping. 



" tetrabedral " groups will be found in the act of splitting 

 along the lines separating the four lobes, and it will be 

 seen that each segment contains a single large spore. 

 This spore is rounded externally, but flattened somewhat 

 on the three internal faces where it was in contact with 

 its sister spores, and its external coat is covered by 

 angular projections. The contents of the upper, simple 

 spore- cases appear as a yellow dust ; but if this is looked 

 at with a microscope it will be found to consist, not of 

 single spores, but of groups of four, arranged in the same 

 " tetrabedral " fashion as the large ones. There are, then, 

 two kinds of spore as well as two kinds of sporangia. 

 The smaller are termed Microspores, and their cases, 

 Microsporanr/iu. The larger are known as Mnrronpores, and, 

 similarly, their enclosing envelopes as Macro-tpurimijia. 



These fern-like plants with two kinds of spores are called 

 lleterosjioroux, while those with only one kmd, such as the 

 true ferns, are known as Homonporoiis. The distinction is 

 not one of merely descriptive value for the purposes of 

 the systematic botanist, but represents a ftmdamental 

 differentiation of the greatest importance. This will be 

 seen clearly if, as in the case of the fern, we follow the 

 germination and resulting growth of these different spores. 



When macrospores have been kept for some time on 

 moist soil, it will be found that the thick protective wall 

 has split, and there is a small projection of soft colourless 

 tissue at the ruptured tip. No green leaf-like cell plates, 

 similar to the fern-prothallus, make their appearance ; yet 

 in time young green seedlings of Selaginella, with up- 

 growing stems, and the characteristic rows of leaves, 

 make their appearance, evidently rising from the macro- 

 spore. The fact that such young Selaginella-sporophytes 

 seem to arise directly from a macrospore suggests two 

 possibilities. Either the Uophyte, or egg-bearing genera- 

 tion, has been entirely lost (in which case we have no 

 explanation of the microspores), or it is so much reduced 

 and concealed as only to be discoverable by careful 

 microscopic investigation. The latter is the true explana- 

 tion. 



Even before the rupture of the spore-wall in germination, 

 the protoplasmic contents of the spore, rich in food 

 materials, will be found to be in part sub-divided by cell 

 walls, forming a definite, if minute and simple, tissue. 

 Similarly, the little colourless papilla which projects from 

 the germinating spore is a distinct cellular outgrowth, and 

 will be found to bear on its margin organs like the arche- 

 gonia of the fern oophyte, though less definite in outline 

 and less complete in structure. ^Ye have, in fact, a small 

 and simplified prothaUus : one that commences its growth 

 within the parent spore ; is fed by the food material con- 

 tained in the spore, never developing roots and green 

 colouring-matter so as to lead an independent existence, 

 but yet containing the egg-cells necessary for the con- 

 tinuation of the plant's being. The diminishing importance 

 of the ouphyte stage observed in the ferns is here carried 

 a step further. The prothaUus is no longer a separate 

 and individual plant, but is reduced to a small, colourless 

 group of cells, living at the expense of the food material 

 stored up in the spore. 



In the fern it will be remembered that all the spores 

 were alike ; that all, on germination, could produce green 

 prothalli ; and that all of these were similar, and pro- 

 duced both archegonia and antheridia. It is true that in 

 some genera prothalli are produced which bear only anthe- 

 ridia, but this seems to be only an abnormal condition, 

 changeable by alteration of surrounding conditions, and 



* The terms Megaspores and Mei/asporangia, recently introduced,; 

 are more classically accurate, but the meaning in this case isidenticaL. 



