THE VEGETABLE KINGDOM 



PLANTS are divided by botanists into four grades, corre- 

 sponding broadly to an advance from an entirely aquatic 

 to an entirely terrestrial life. From* what we have 

 learned from fossil animals we may expect that such an 

 advance would not take place along one line but along 

 several parallel lines, and that there would be occasional 

 reversions to an aquatic life on the part of plants whose 

 ancestors had become adapted to the land. Both these 

 expectations are justified. 



1. Thallophyta. This, the lowest grade, corresponds 

 in a general way to the Protozoa among animals, its 

 members being composed of cells only, though they may 

 form aggregates of much greater size and more dif- 

 ferentiation than do the Protozoa. An alternation of 

 sexual and asexual generations (as in the Foraminifera 

 and other Protozoa) exists, and is important for its 

 bearing on the life-history of the higher plants. The 

 Thallophyta include the Algae (the great majority of all 

 aquatic plants) and the Fungi. 



2. Bryophyta. In this grade (mosses and liverworts) 

 the alternation of generations is much more regular 

 than in the Thallophyta, the sexual generation (game? 



