60 EVOLUTION OF PLANTS 
a single chromatophore, and the cell may be compared 
structurally to that of the Protococcacee or Volvocinez, 
and it is probably from these that the lower Conjugate 
have arisen. Reproduction takes place in the lower des- 
mids either by the division of the cells (Fig. 18, 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 zodspores within the resting-spores of many Pro- 
tococcaceee and Confervacez. 
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 oné 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 Mesotenium closely resembles 
the individual cell of the filamentous Mesocarpus, and 
Spirotenia bears the same resemblance to Spirogyra. 
The chloroplasts of the Conjugate 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 alge (see Fig. 18). 
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- 
