324 Science of Plant Life 



creasingly more complex from the ferns to the flowering giants ; 

 but the cells of which the bundles are composed are very 

 similar. The reproductive structures also show a gradual 

 modification in plants. They are comparatively simple in 

 the ferns and very complex in the flowering plants, and be- 

 tween the two extremes there are many gradations. These 

 gradations occur in such an order that they point to the grad- 

 ual coming into existence of complex reproductive structures, 

 culminating in those of insect-pollinated flowers. 



Life histories of plants. Again, when we compare the de- 

 tailed life histories of plants belonging to the different groups, 

 the close resemblances among them point to their origin by 

 gradual evolution from common ancestors. As one passes 

 from the algae through the mosses and ferns to the Gymno- 

 sperms and Angiosperms, the life histories of the plants, like 

 their structures, become more and more complicated. This 

 increase in complexity of the life history is brought about by 

 gradual modification and by the addition of new steps and 

 new structures. The life histories of related groups are 

 similar in essentials and differ only in details. This repetition 

 of the stages in the life cycles of the plants of different groups 

 can be explained only by assuming that the plants with the 

 more complex life histories have evolved from those with less 

 complex life histories. Increase in complexity is one of the 

 general tendencies of evolution. The order in which we should 

 arrange plants on the basis of the geological records is the same 

 as the order suggested by their life histories and structures. 



Intergrading species. All who have attempted to classify 

 plants — that is, to determine the species to which individual 

 specimens belong — have been impressed by the intergrading 

 of related species. The existence of individuals intermediate 



