332 PRACTICAL BOTANY 



The green algae offer the best series in evolution of repro- 

 ductive organs. One-celled plants that reproduce vegetatively 

 (fission) and sometimes by zoospores are followed by plants 

 which, while still reproducing vegetatively, have vegetative 

 cells which, by division of their contents, produce numerous 

 asexual spores (zoospores), as in Cladophora and Ulothrix. 

 From small zoospore-like bodies the first sex spore (zygospore) 

 is formed, and the origin of gametes and sexuality in plants 

 appears. Then in (Edogonium vegetative cells produce special 

 sex organs (oogonia and antheridia) in which differentiated 

 gametes (eggs and sperms) are formed. From these the oospore 

 is formed. In such cases as this we have an illustration of the 

 differentiation of sex cells into male and female, and the dif- 

 ferentiation of sex organs. Finally, in plants such as Vaucheria 

 the sex organs are made solely for reproductive work. 



304. Bryophytes. This division is subdivided into two 

 classes, liverworts and mosses. The protonema of mosses 

 greatly resembles the green algae, and some simple liver- 

 worts are masses of cells much like some of the higher algae. 

 Rhizoids and special chlorophyll tissues are developed both in 

 liverworts and in mosses, and in the highest members of each 

 class leaf -like and stem-like structures are formed. In the 

 mosses this leafy plant is erect, the leaves are radially arranged 

 about the stems, and altogether the mosses appear to be good 

 chlorophyll-working plants. While the liverworts have not 

 developed the erect stem and radially arranged leaves, in the 

 leafy liverworts there is an almost equal degree of vegetative 

 differentiation. The chlorophyll tissues of bryophytes present 

 a great advance over those of thallophytes. 



In reproduction, bryophytes also offer distinct advances over 

 thallophytes. The sex organs are complex structures. The 

 egg is produced in a many-celled archegonium, instead of the 

 one-celled oogonium of the thallophytes, and the sperms are 

 produced in a many-celled antheridium. These organs may 

 be embedded in the thallus or borne upon the surface. The 

 oospore produces a distinct phase of the plant, which when 



