106 BOTANY 



complexity of the sporocarp it came to have an inde- 

 pendent existence, and that when it had achieved this, 

 it became an erect plant, the conditions we have already 

 alluded to making this necessary. At some stage or 

 other in the development a difference arose among the 

 prothalli arising from the spores, some giving rise to 

 sperms only while the others only produced ova. Gradu- 

 ally the change spread to the spores themselves, those 

 giving rise to sperm-bearing prothalli remaining small, 



FIG. 42. Section of sorus of fern showing sporangia 

 covered by an indusium. X5o. (After Kny.) 



those producing ova-bearing prothalli becoming much 

 larger. The prothalli also changed. Those from the 

 small spores became in some cases filamentous, and in 

 all extremely minute, consisting of little more than the 

 organ giving rise to the sperms. Those from the large 

 spores only protruded partly from the ruptured spore, 

 and came to be developed almost entirely inside it, the 

 spore splitting its coats but slightly. Such prothalli 

 developed very few archegonia. 



These forms are still represented to-day by the 

 Selaginellas, a group classed among the fern-like plants. 



