ALTERNATION OF GENERATIONS 485 



Liverworts also show this ; but in their larger forms the upper surface 

 of the thallus may be alveolated, and the cavities occupied by Photo- 

 synthetic tissue, so as to make them efficient for self-nutrition in 

 dry air, as in the leaves of Flowering Plants (p. 365). In the 

 Mosses and leafy Liverworts, after a preliminary filamentous stage, 

 a leafy plant is formed after the fashion of the leafy sporophyte. 

 . In 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 Photo-Synthesis combined with water-control (p. 357, 358). 

 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 absence of internal ventilation. Air- 

 containing intercellular spaces are wanting. The plant is essentially 

 semi-aquatic, and often saves itself in its land-habitat, as the Mosses do, 

 by its power of dormant vitality under drought, and its readiness of 

 surface-absorption whenever water is available. Thus constituted the 

 gametophyte is a constant menace to success of the Archegoniatae, 

 as Land-living Plants. Its delicate structure, and, finally, its 

 dependence on external fluid water for fertilisation, have 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 

 mycorhiza, and saprophytic nutrition (Chapter XI.). It must suffice here to 

 note the fact 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 the Bryophytes, homosporous, with all the spores alike, and 

 commonly yielding on germination a bi-sexual prothallus (p. 342, Fig. 

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

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



