ALTERNATION OF GENERATIONS 5; 



sporogonia of these possess stomata and internal ventilation, thou 

 frequently their stomata are imperfect and functionless. Such str 

 ture suggests for them a descending rather than an ascending scalt 



vegetative development. If this view be correct it will not be among 



the smallest and structurally simplest BryophyU-s tl | e should 

 seek for the key to the early history of the sporophyte in Archegoniate 

 Plants. 



A most important feature of the Land Vegetation is the retention 

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

 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 

 the constancy of form and structure of the archegonium 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 recep- 

 tive period. The immediate consequences of this retention of the 

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

 spermic 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 liquid 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 its 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 even be filamentous, while the Alga-like habit n. 

 be accentuated in vegetative propagation by gemmae. The thalloid 

 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. 472, Fig. 368). In the 

 Mosses and leafy Liverworts, after a preliminary filamentous staj 

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



