10 EVOLUTION OF PLANTS 



and structure, and constitute tissues of different kinds 

 which are in turn aggregated to form special organs. 

 Thus a special subterranean root system is present, and 

 the green assimilative tissue is mainly confined to the 

 leaves, which are preeminently organs for carbon assimi- 

 lation. The extreme of specialization is reached in the 

 flowers of these plants, which are beyond doubt the 

 most complicated structures which occur in the plant 

 kingdom. 



Between the extremes found in the unicellular plants 

 at the bottom of the series, and the complicated seed 

 plants at the top, are numberless gradations of structure 

 which throw much light upon how these advances in 

 structure have been brought about. A similar progress 

 from the simple to the complex is, of course, evident in 

 the evolution of the animal kingdom ; but the animal 

 type reaches a far greater degree of complexity and 

 specialization than is ever found even in the highest 

 plants, which differ much less from the lower ones than 

 is the case among animals. 



At the bottom of the scale the two kingdoms con- 

 verge. There are many forms to be met with whose 

 position is more or less doubtful, and in some cases it 

 is practically impossible to determine to which great 

 division they belong. We can only say that we have 

 to do with organisms which are not yet sufScientl}^ 

 differentiated to determine whether the animal or 

 vegetable characters predominate. It is the study of 

 these primitive organisms, and the realization of the 

 close similarity in the structure and functions of the 

 animal and plant cell, which emphasize the intimate 

 connection between the two kingdoms, and the impos- 



