108 EVOLUTION OF PLANTS 



conditions, as well as their abundance and variety, 

 indicate a more modern type than the thallose forms 

 with which they are connected by various intermediate 

 conditions (Fig. 27, D). 



Some of the foliose Hepaticse, especially certain tropi- 

 cal types, show extremely curious modifications of the 

 leaves to form reservoirs of moisture or even traps for 

 small Crustacea, recalling those found in some flower- 

 ing plants, such as the bladder-weed (Utricularia). 



The range of structure in the sporophyte of the He- 

 paticse is great, and a study of the different types is 

 most instructive in showing the growing importance of 

 the sporophyte in passing from the lower forms to those 

 which approximate the structure of the higher plants. 



The simplest sporophyte is met with in the genus Ric- 

 cia (Fig. 27, A, Fig. 28, B), which comprises a number 

 of small thallose liverworts, where there is no trace 

 of any differentiation of the gametophyte into stem 

 and leaves ; but the thallus is not so primitive as in 

 certain other forms which have a more highly developed 

 sporophyte. The sexual organs are borne upon the 

 dorsal surface of the gametophyte, but not arranged 

 in any definite order. They have the typical structure 

 found in other Hepaticse. The archegonium (Fig. 26, 

 B) contains the egg in the enlarged ventral portion, 

 and when the plants are covered with water, it opens 

 and allows the sperm atozoids, which have at the same 

 time been liberated from the ripe antheridium, to swim 

 into it. The spermatozoid penetrates the egg-cell, 

 which thereupon is stimulated into active growth, and 

 develops into the sporophyte, or sporogonium, as it is 

 commonly termed in the mosses. The development of 



