134 



NATURAL SCIENCE. 



borne on the same leaf, though this has become considerably 

 modified and differs remarkably in appearance from the vegetative 

 leaf. 



The prothallium resulting from the microspore is very simple. 

 In Pilulana, for instance, the spore divides into three cells, the 

 smallest representing the prothallium, while the other two together 

 form the antheridium, and, by repeated bipartition, give rise to six- 

 teen naked antherozooids. The prothallium and antheridium remain 

 enclosed in the spore, which ultimately bursts to allow the escape of the 

 antherozooids. The macrosporangium, like the microsporangium, 

 becomes separated from the leafy asexual plant, but the macrospore 

 never gets quite free, although in course of growth it ruptures the 

 sporangium wall above. 



The female prothallium, like the male, is very simple and 

 contained in the spore, forming a small green structure on which one 

 or more archegonia are developed in the same way as in the homo- 

 sporous Ferns. The sporangium and spore become ruptured near the 

 apex to give access to the antherozooids. 



We may represent the life history thus : — 



Stem- and leaf-bearing plant 



Asexual generation 



Sexual generation 



M icrosporangium 



Microspores 



I 

 Male prothallium 



Antheridium 



Antherozooids 



Macrosporangiu m 



I 

 Macrospore 



lemale prothallium 



Archegonium 



I 

 Oosphere 



Oospore 

 New stem- and leaf-bearing plant. 



Besides the homosporous and heterosporous ferns, the group 

 Vascular Cryptogams comprises also the Equisetineae, or Horse-tails, 

 and the Lycopodineae, including the Lycopodiacea',or Lycopods proper 

 (Club-mosses), and the Ligulateae, comprising Selaginella and Iso'ctes 

 (the British Quiliwort). Of these the Equisetineae and Lycopodiaceae 

 are homosporous, while Selaginella and Iso'ctes are heterosporous. The 

 sporangia are produced, not from a single cell, as in the Ferns, but 

 from a group, which forms a hemispherical protuberance on the 

 under margin of the modified, peltate scale-leaf of the Equisetineae, 

 or in the upper surface of the base of the leaf in the Lycopods, in the 

 hollow of the leaf-sheath of Iso'ctes, or on the stem just above the leaf- 

 base of Selaginella. The archesporium is either a single cell beneath 

 the epidermis, as in Equisetineae, or a hypodermal layer, as in 

 Isoetes. 



If we follow the process in the macrosporangium of Isoetes, we find 

 that the cells of the archesporium elongate and divide by transverse 



