20 EVOLUTION OF PLANTS 



the cell increases in bulk, and sooner or later divides 

 to form new ones which become at once new individuals, 

 so that the single cell exhibits all the characteristics of 

 a typical plant; i.e. it feeds, grows, and reproduces 

 itself, and all these functions are performed by one 

 and the same cell. Such a unicellular plant cannot 

 properly be considered as strictly undifferentiated, 

 since the permanent constituents of the cell, e.g. 

 nucleus and chromatophore, must be regarded as defi- 

 nite organs. 



As we pass from the unicellular plants to the simpler 

 multicellular forms (see Fig. 9, F) we find the first 

 indications of a specialization of certain cells. Thus the 

 basal cell has very little chlorophyll, but is modified 

 into a root-like organ for the attachment of the plant, 

 while the other cells with their numerous chloroplasts 

 are alone concerned with the nutrition of the plant. 

 Very much more complicated are many of the large 

 sea-weeds, some of which, like the great kelps (see 

 Fig. 17), reach an enormous size. In these plants 

 there are various sorts of cells aggregated into definite 

 organs. The chloroplasts are mainly confined to the 

 outer part of the plant, where they may be fully exposed 

 to the light, while the inner tissue has little or no 

 chlorophyll, and the cells are modified for conducting 

 purposes. In the most highly organized of these marine 

 algse, like the gulf-weed (Fig. 18), a further advance 

 is seen in the formation of flattened leaves to which 

 the chlorophyll-bearing cells are mainly restricted. 

 Indeed these highly specialized sea-weeds bear a most 

 remarkable superficial resemblance to the flowering 

 plants in the development of a definite branching axis 



