484 BOTANY OF THE LIVING PLANT 



Presumably it was from some Algal ancestry in which sexuality was 

 already established, and perhaps it already possessed a definite alterna- 

 tion of generations. It cannot be asserted what the balance of those 

 generations was. But judging from the constancy of well-marked 

 alternation of x and 2x generations in all land-living forms, and the 

 fact that such alternation is also present in Algae, both generations 

 may have been already in existence as somatic structures before the* 

 transition to Land-Life. This, however, appears to have fixed, and 

 indeed differentiated the two generations : so that the Bryophyta 

 and Pteridophyta now appear as the headquarters of well-marked 

 alternation of generations with distinctive characters of form and 

 structure. 



A most important feature of the Land-Vegetation is the retention 

 of the ovum within the parent plant. It is enveloped in the arche- 

 gonium. The archegonium itself is so constant in the earlier Land- 

 . Vegetation that on it is based the name " Archegoniatae," so often 

 applied collectively to the Mosses and Ferns. The explanation of 

 its constancy of -form and structure, though not of segmentation, is 

 to be found in the imperative need for the protection which it offers 

 to the ovum, but without excluding access of the spermatozoid at 

 the receptive period. The immediate consequences of this retention 

 of the ovum are seen in the fact that Archegoniate Plants, or their 

 Gymnospermic derivatives, form the bulk of the early Fossil Flora, and 

 are an integral part of the Land-Flora of the present day. But such 

 organisms have not cut themselves wholly adrift from their original 

 mode of life. They are still dependent upon external fluid water for the 

 fertilising act itself, since it is through water that the male gamete 

 moves to the egg. Moreover, the gametophyte with its relatively 

 delicate structure is essentially dependent upon moist conditions for 

 its normal growth. 



RISE AND DECADENCE OF THE GAMETOPHYTE. 



The gametophyte has never made a real success of Life on Land, 

 as measured by size and structure. But this in itself makes the 

 study of its partial success the more interesting. In Ferns and the 

 thalloid Liverworts it is commonly a flattened thin or fleshy body 

 of undifferentiated tissue, capable of self-nourishment and absorption 

 from the soil. In extreme cases, growing in very moist and shaded 

 conditions, it may be even filamentous, while the Alga-like habit is 

 accentuated by vegetative propagation by gemmae. The thalloid 





