VASCULAR CRYPTOGAMS. 037 



in the Selaginellese), enveloping the embryo, until this latter finally protrudes free, 

 leaving, however, for some time, the foot still attached to it as a nutritive organ. 

 This process offers an unquestionable analogy to the formation of the calyptra 

 of the IMuscineae. While, however, the spore-producing generation of the Mus- 

 cineae remains a mere appendage of the sexual plant, appearing, in a certain 

 sense, as its fruit, the corresponding generation of Vascular Cryptogams developes, 

 on the contrary, into a conspicuous, highly organised, independent plant, which 

 frees itself at a very early period from the prothallium, and obtains its own 

 nourishment. It is this asexual generation which is called, in ordinary language, 

 simply the Fern, Equisetum, &c. ; it always consists of a leafy stem, usually pro- 

 ducing a number of true roots ; roots may, however, occasionally be entirely 

 absent, as in some species of Hymenophyllum, and in Psilotum and Salvinia. 

 In many cases, especially in Ferns, Equisetaceae, and (especially the extinct) 

 Lycopodiaccae, the spore-producing generation attains great dimensions with an 

 unlimited term of life ; only a few species are (like Salvinia) annual. 



The Leaves are either simple, unsegmented, or variously branched (Ferns, 

 Ophioglossaceae). There does not, however, occur so great a variety in the forms 

 assumed by the leaves in the same plant due to metamorphosis as in Phanerogams. 



The Roots usually arise in acropetal succession on the stem (or on the leaf- 

 stalk in some Ferns), and branch monopodially or dichotomously ; they always 

 remain nearly uniform in size, the first root never attaining the dimensions of a 

 tap-root, as in many Phanerogams. 



The Differentiation of the Systems of Tissue attains a high degree of perfection 

 for the first time in this group of plants. The epidermis, fundamental tissue, and 

 fibro-vascular bundles are always clearly distinct, and are composed of cells of 

 various forms. The fibro-vascular bundles are closed; their phloem usually sur- 

 rounds the xylem of each separate bundle like a sheath. 



The Bra?iehi?ig of the Stem is very different in the different classes of 

 Vascular Cryptogams, and will be considered hereafter; it may be remarked here 

 that axillary branching probably does not occur in the same sense in which the term 

 is applied to Phanerogams. 



The Production of the Sporangia is, in most cases, evidently a function of the 

 leaves ; in a few cases (as Pilularia) this mode of origin is, however, still doubtful. 

 In their form and mode of envelopment by neighbouring organs the sporangia show 

 considerable differences ; but within each class their characters are very constant. 



It is clear from what has now been said that the sporangium of Vascular 

 Cryptogams is equivalent, from a physiological but not from a morphological point 

 of view, to the sporogonium of Mosses. This latter forms by itself the whole of the 

 asexual generation of Mosses ; while the sporangium of Vascular Cryptogams is a 

 relatively small outgrowth of a foliar structure of the asexual generation which 

 consists of stem, leaf, and root. The mode of origin of the mother-cells of the 

 spores is also different from that in the Muscineae, though the spores themselves 

 are produced in the mother-cells in a manner more like that which occurs in 

 Muscinese. The spore-mother-cells of Vascular Cryptogams also become isolated 

 from their combination into a tissue, and divide into four spores, an indication of 

 a division into two generally preceding that into four. The distinction between 



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