232 PTERIDOPHYTA. 



The FERTILE LEAVES most frequently differ from the barren ones, 

 and are collected into spike-like cones (a kind of flower; Fig. 239). 

 Micro- and macro-sporangia are found in the same cone (Fig. 239). 

 Each sporangium arises from a group of superficial cells of the 

 stem, directly over the leaf on which it will be situated later on. 

 Each sporangium has a hypodermal, unicellular archesporium, 

 and contains a layer of tapetal cells ; these are dissolved later, 

 when the spores are ripe, and not before as in the Ferns. In the 

 very early stages of their development, 

 the micro- arid macro-sporangia are 

 precisely similar, and the differences 

 between them arise later on. In the 

 microsporangium all the spore-mother- 

 cells divide, and each forms four tetra- 

 hedrically -arranged microspores (Fig. 

 204) ; but in the macrosporangium 

 only four macrospores are formed, by 

 the division of a single mother-cell, 

 while the remaining spore-mother-cells 

 are aborted. It is rarely that the 

 macrosporanjria contain 2 or 8 macro- 



FIG. 2-10. Selaginella martensii: 

 K lower leaves ; r upper leaves. spores. 



For the GERMINATION OF THE SPORES, see pages 228, 229. The protballium 

 arises in the macrospore (/-/, in Fig.235^i), probably by division of the meniscus- 

 shaped protoplasmic mass, which is marked off at the apex of the spore ; 

 primordial cells are thus formed which later on are surrounded by a cell-wall. 

 In six to seven weeks after sowing, the spore-wall is ruptured by the growing 

 prothallium, which already has developed archegonia (Fig. 235 ce-ce). The 

 prothallium so formed does not occupy the entire cavity of the spore, but ftfur 

 to five weeks after sowing, the large- celled parenchyma is developed in the 

 lower portion of the spore by free cell-formation; this has been termed by Pfeffer, 

 " endosperm." since it is similar to the endosperm of Flowering-plants. Goebel, 

 however, has termed it " secondary prothallium," as the homology with the 

 endosperm of the Angiosperms is very doubtful. 



The FERTILISED OOSPHERE divides into an upper (hypobasal) and 

 a lower (epibasal) cell; from the latter alone the embryo is deve- 

 loped with its root, stem, foot, and two cotyledons, and the former 

 gives rise to an organ which appears in this instance for the first 

 time, but which occurs in all Flowering-plants, viz. the suspensor. 

 This forces the embryo down into the " endosperm," which is 

 entirely or partially absorbed by the embryo. In the case of the 

 Flowering-plants the embryo is developed with its longitudinal 



