112 EVOLUTION OF PLANTS 



capsule is ripe, help to distribute the spores (Fig. 

 28, H). 



In contrast to the simple sporophyte of the lower 

 liverworts, there is found in one group a sporophyte 

 which reaches a high degree of complexity, and be- 

 comes almost independent of the gametophyte. This 

 reaches its highest expression in the genus Anthoceros 

 (Fig. 28, D, Fig. 31, C). Here the gametophyte is very 

 primitive and consists of a simple thallus composed 

 of almost perfectly uniform cells, and without any dif- 

 ferentiation into stem and leaves. Indeed, it represents 

 almost the lowest type of the gametophyte among the 

 Hepaticse. A suggestion of an origin of this type of 

 thallus from the Algse is seen in the single chloroplast 

 in each cell, much like that in Coleochsete. The sexual 

 organs of Anthoceros, while on the whole like those of 

 the other liverworts, are peculiar in being sunk in the 

 thallus, and recall, in this respect, those of the more 

 primitive ferns. 



It is the sporophyte, however, which is of the greatest 

 interest. This reaches a relatively large size (Fig. 31, 

 C, sp) and shows a considerable degree of independent 

 growth. Between the large foot and the upper portion 

 is a zone of growing tissue, which enables the sporophyte 

 to grow in length as long as the gametophyte remains 

 active, and from this growing zone new tissue is con- 

 stantly added to the base of the sporophyte. The latter 

 has its outer parts developed into a perfect assimilating 

 tissue with several layers of spongy green tissue whose 

 air-spaces communicate with the outside atmosphere by 

 means of stomata or pores in the epidermis, precisely 

 like those found upon the leaves of the higher plants. 



