CHAPTER XLIII. 



POLLINATION. 



Origin of heterospory, and the necessity for 

 pollination. 



835. Both kind*-, of sexual organs on the same prothallium. In the ferns, as 

 we have seen, the sexual organs are borne on the prothallium, a small, leaf-like, 

 heart-shaped body growing in moist situations. In a great many cases both 

 kinds of sexual organs are borne on the same prothallium. While it is per- 

 haps riot uncommon, in some species, that the egg cell in an archegonium 

 may be fertilized by a spermatozoid from an antheridium on the same pro- 

 thallium, it happens many times that it is fertilized by a spermatozoid from 

 another prothallium. This may be accomplished in several ways. In the 

 first place antheridia are usually found much earlier on the prothallium than 

 are the archegonia. When these antheridia are ripe, the spermatozoids es- 

 cape before the archegonia on the same prothallium are mature. 



836. Cross fertilization in monoecious prothallia. By swimming about in 

 the water or drops of moisture which are at times present in these moist situa- 

 tions, these spermatozoids may reach and fertilize an egg which is ripe 

 in an archegonium borne on another and older prothallium. In this way 

 what is termed cross fertilization is brought about nearly as effectually as if 

 the prothallia were dioecious, i.e. if the antheridia and archegonia were all 

 borne on separate prothallia. 



837. Tendency toward dioecious prothallia. In other cases some fern pro- 

 thallia bear chiefly archegonia, while others bear only antheridia. In these 

 cases cross fertilization is enforced because of this separation of the sexual 

 organs on different prothallia. These different prothallia, the male and 

 female, are largely due to a difference in food supply, as has been clearly 

 proven by experiment. 



838. The two kinds of sexual organs on different prothallia. In the horse- 

 tails (equisetum) the separation of the sexual organs on different prothallia has 

 become quite constant. Although all the spores are alike, so far as we can 

 determine, some produce small male plants exclusively, while others produce 



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