150 STCDT OF COMMON PLANTS. 



or an expanded leaf-like body, as in various marine species, 

 or it maj- take still other forms in considerable variety. 

 It is in any case a cellular structure, the cells of which it 

 is composed being comparatively little modified, and char- 

 acterized by the presence of chlorophyll. The thallus of 

 liverworts shows considerable advance beyond the algae, 

 corresponding with its adaptation to terrestrial conditions, 

 shown in the formation of mechanical cells, the presence of 

 stomata, and in other structural features ; and the mosses, 

 with their usually manifest differentiation into root-like or- 

 gans, stem, and leaf, exhibit in some respects a still higher 

 development. Both mosses and liver worte, however, arise 

 from a protonema, and in this fact point to their unmis- 

 takable relationship with simple alga forms. In the ferns, 

 and in general in the vascular cryptogams, the vegetative 

 body consists of a pro thallium and of the more conspicu- 

 ous leafy plant, the latter distinguished by the formation 

 of vascular tissue and a higher degree of differentiation 

 than is characteristic of cellular plants. The prothallium 

 is usually a flat, cellular structure, reminding one of the 

 thallus of a liverwort, and bearing the archegonia and 

 antheridia. It is of special interest to notice that just as 

 the affinities of liverworts and mosses with the algse are 

 indicated by the structure of the protonema, so those of 

 the vascular crj-ptogams with the mosses and their allies 

 are suggested by that of the prothallium. 



The spores, to which in this review our attention was 

 first directed, are produced in cells of the vegetative body 

 of the plant, or as direct outgrowths from it, or 4, ^ , 

 upon special structures called sporophores. They generations. 

 are non-sexual or sexual in their origin. The sporangio- 

 spores of Rhizopus are an example of the former, the 

 oospores of Vaucheria of the latter, class. While in the 



