24 STRUCTURAL BOTANY 



ably corresponds to the first two cells cut off in the 

 pollen-grain of Picea (see Part I. Fig. 108, cells marked 

 2 and 3, p. 267). In neither case do these cells take 

 any part in the further development. The succeeding 

 divisions are more numerous in Selaginella than in 

 our gymnospermous type. The whole resulting group, 

 including both enveloping and central cells, constitutes 

 an antheridium, the characteristic male organ of the 

 Cryptogams. In the higher Cryptogams, this organ 

 always consists of an enveloping layer of cells enclosing 

 the central group from which the spermatozoids are 

 derived. In the Gymnosperms the antheridium is 

 only represented by the stalk-cell and the two genera- 

 tive cells (see Part I. Fig. 108, 4 and 5, p. 267). The 

 whole is enclosed in the undifferentiated vegetative 

 cell, which forms the bulk of the pollen-grain. In 

 Selaginella the vegetative cell ceases to exist ; it is all 

 used up in forming the antheridium, while in Gymno- 

 sperms it persists in order to produce the pollen-tube. 



We see, then, that microspores and 'pollen-grains, 

 which agree exactly in their mode of origin, agree also 

 up to a certain point in their mode of germination. 

 The differences between them are connected with the 

 different means by which fertilisation is effected. 



c. Germination of the Megaspores 



Unlike the microspores, the megaspores of Selaginella 

 begin to germinate while still in the sporangium. 

 Each of the four megaspores is tetrahedral in shape, 

 like a microspore. It contains at first a single nucleus 

 and abundant protoplasm, in which is a large vacuole 

 containing oil. The nucleus lies near the angle, where 

 the megaspore joins its three sister- cells. We will 



