550 BOTANY OF THE LIVING PLANT 



the larger forms it may develop a conducting system, while some- 

 times, by involution of their surface, its leaves may acquire a structure 

 efficient for photosynthesis combined with water-control (pp. 464, 

 465). But the size of these gametophytes is never great, and often 

 very minute. Even in its most successful forms the sexual generation 

 suffers from the disability of an imperfect internal ventilation. It is 

 essentially semi aquatic, and often saves itself, as the Mosses do, by its 

 power of dormant vitality under drought, and its readiness in surface- 

 absorption whenever water is available. Thus constituted the gameto- 

 phyte is a constant menace to the success of the Archegoniatae, as 

 Land-living Plants. Finally, its dependence on external water for 

 fertilisation has tended to tie the lower Archegoniatae down to limited 

 habitats, from which they have never been fully emancipated. 



Some of the most archaic plants that have survived, such as the Psilotaceae, 

 Lycopods, and Ophioglossaceae, have underground prothalli with endotrophic 

 mycorrhiza, and saprophytic nutrition (Chapter XII.) Incidentally it may 

 be noted that internal ventilation is absent from these prothalli, even when 

 their size is great. This is quite exceptional. It must suffice here to state 

 these facts without detailed description. In view of the disabilities of the 

 gametophyte for life on land, the underground habit and the form of sapro- 

 phytic nutrition which these plants possess may well have been conditions 

 which have determined their survival. 



The difficulty presented by this dependence of the gametophyte 

 upon external water has been met in the Higher Flowering Plants by 

 a repetition of the method already so successful in the first conquest 

 of the Land, viz. the retention of the vulnerable part upon the parent. 

 First the ovum was retained, as in the Archegoniatae, then the whole 

 prothallus which bears it, as in the Seed-bearing Plants. For this 

 the way was prepared by the sexual differentiation of the spores. 

 Within each of the phyla of Ferns, Lycopods, and Equiseta, this 

 differentiation has taken place. In each case the original state was, 

 as in all Bryophytes, homosporous, with all the spores alike, and 

 commonly yielding on germination a bi-sexual prothallus (p. 500, Fig. 

 392). The first step is a separation of the sexes on distinct prothalli. A 

 purely male prothallus has no permanent duty, but only the temporary 

 function of producing spermatozoids. It may therefore, and it does, 

 remain small. But the female prothallus has both to produce ova 

 and to nourish the embryo after fertilisation. This can best be 

 carried out by a large prothallus, which will develop better from a 

 well-nourished spore. This is the physiological rationale of the origin 

 of the megaspore as distinct from the microspore, as seen in Selaginella, 



