14 THE MORPHOLOGY OF PTERIDOPHYTES 



dependent on it for nutrition. By contrast, among pterido- 

 phytes the sporophyte is the dominant generation, for it very 

 soon becomes independent of the gametophyte (prothallus) 

 and grows to a much greater size. Along with greater size 

 is found a much greater degree of morphological and 

 anatomical complexity, for the sporophyte is organized into 

 stems, leaves and (except in the most ancient fossil pterido- 

 phytes and the most primitive Uving members of the group) 

 roots. Only the sporophyte shows any appreciable develop- 

 ment of conducting tissues (xylem and phloem), for although 

 there are recorded instances of such tissues in gametophytes, 

 they are rare and the amounts of xylem and phloem are 

 scanty. Furthermore, the aerial parts of the sporophyte are 

 enveloped in a cuticle in which there are stomata, giving 

 access to complex aerating passages that penetrate between 

 the photosynthetic pahsade and mesophyll cells of the leaf. 

 All these anatomical complexities confer on the sporo- 

 phyte the potentiaUty to exist under a much wider range of 

 environmental conditions than the gametophyte. However, 

 in many pteridophytes these potentialities cannot be realized, 

 for the sporophyte is limited to those habitats in which the 

 gametophyte can survive long enough for fertilization to 

 take place. This is a severe hmitation on those species whose 

 gametophytes are thin plates of cells that lack a cuticle and 

 are, therefore, susceptible to dehydration. Not all gameto- 

 phytes, however, are Umited in this way, for in some pterido- 

 phytes they are subterranean and in others they are retained 

 within the resistant wall of the spore and are thus able to 

 survive in a much wider range of habitats. It is notable that 

 wherever the gametophyte is retained within the spore the 

 spores are of different sizes (heterosporous), the larger 

 megaspores giving rise to female prothalH which bear only 

 archegonia, and the smaller microspores giving rise to male 

 prothaUi bearing only antheridia. Why this should be is not 

 known with certainty, but two possible reasons come to 

 mind, both of which probably operate together. 



