166 



PLANT-LIFE 



megaspores in each, and are the larger. Both kinds 

 occur in the same cone. 



Very little water is necessary to enable the micro- 

 spores to germinate. The development is illustrated 

 in Fig. 55, from which it will be seen the prothallus 

 consists of a single prothallus cell, and an antheridium 

 in which spermatozoids are formed. The spermatozoids 

 are exceedingly small ; they are furnished with two cilia, 



with which the water is 

 lashed in swimming. It 

 sp. m. is a noteworthy fact that 

 the megaspores begin 

 their germination while 

 they are still in their 

 sporangia, but it is not 

 completed until they fall 



to the ground, yet before 

 Fig. 55. — Germinating Microspores .-, . -, 71 



of Selaginella. this happens a small pro- 



A, Early stage, x 290; p, prothallus- thallus and an arche- 



cell; B, farther advanced stage, gonium may be formed 

 X 290 ; p, prothallus-cell ; c, cells -,-, • .-, A <• , 



which will produce spermatozoids: mthlU the S P 0Ie - After 



C, mature stage, x 640; sp.m., sper- reaching the ground the 



matozoid mother-cells; D, free sper- ^ f ^ megaspore 

 matozoids, x 780. & r 



is ruptured, the prothal- 

 lus grows to a very limited extent, and more arche- 

 gonia appear round the original one. An archegonium 

 ripe for fertilization is seen in Fig. 56. Fertilization is 

 effected by swimming spermatozoids, which are attracted 

 by archegonia that are sufficiently near. The male 

 elements are so small that the veriest film of water on 

 the surface of damp ground is sufficient to enable them 

 to swim freely. The fertilized ovum becomes an 



