60 EVOLUTION OF PLANTS 



a single chromatophore, and the cell may be compared 

 structurally to that of the Protococcacese or Volvocinese, 

 and it is probably from these that the lower Conjugatse 

 have arisen. Reproduction takes place in the lower des- 

 mids either by the division of the cells (Fig. 13, F), or 

 by the fusion of two of them into a single cell, or spore, 

 which subsequently by internal division gives rise to 

 several new individuals very much like the production 

 of zoospores within the resting-spores of many Pro- 

 tococcacese and Confervacese. 



From these simple unicellular types, it is easy to trace 

 the development of the series in one direction, by 

 specialization of the individual cells, to the higher 

 desmids ; in the other, by cohesion of the cells, to the 

 filamentous pond-scums. The latter, probably, do not 

 all form one group, but have originated from several 

 types of unicellular ancestors, as there are several genera 

 of unicellular desmids, which, in the form of their pe- 

 culiar chloroplasts, closely resemble the different genera 

 of the pond-scums. Thus Mesotsenium closely resembles 

 the individual cell of the filamentous Mesocarpus, and 

 Spirotsenia bears the same resemblance to Spirogyra. 



The chloroplasts of the Conjugatse are always large 

 and usually have the form of a flattened band or plate 

 in which are imbedded one or more roundish bodies, 

 pyrenoids, such as are common in the chloroplasts of 

 most other green algae (see Fig. 13). 



The absence of motile reproductive cells necessitates 

 a special contrivance for fertilization. Except in a few 

 of the lowest forms where the unicellular individuals 

 fuse together completely, union of the sexual cells is 

 accomplished by the formation of protuberances, grow- 



