HIGHER AND LOWER PLANTS 269 



each of the three horizontal planes, and even in individual orders, 

 in their lowest and highest plants. 1 To facilitate the compre- 

 hension of this classification I have assembled a sufficient 

 number of the plants themselves, so arranged as to place be- 

 fore you a living representation of this complicated diagram. 



The laws controlling the chemical evolution of plant -con- 

 stituents are too little comprehended to formulate, but before 

 reaching a position ever to do this, it will be necessary to study 

 carefully the facts from extended researches, to ascertain how 

 these chemical constituents occur, under what conditions, and 

 if these conditions are constant or variable, and to what may 

 be ascribed the variability. 



In speaking of chemical compounds I will describe them as 

 occurring according to the botanical disposition of Heckel's 

 table, which I use provisionally, since it is not probable that 

 this presentation will be the ultimate or best way to introduce 

 the subject. But I am not prepared as yet to offer any other 

 arrangement on a purely chemical basis; since the application 

 of the chemical side of plant life as one more evidence in favor 

 of the hypothesis of evolution is still too new to possess a litera- 

 ture of its own. 



I have already referred to the protoplasm and starch, also to 

 the large ash-percentages of some of the lower groups, and 

 among the compounds commonly found in many plants, tannin 

 appears first, according to the evolutionary order, in liverworts. 



Chlorophyll is one of the earliest compounds to appear, and 

 its presence in Algae and its absence in Fungi is a distinction 

 between the two divisions of the Thallophyta group. Besides 

 this green coloring-matter, which is, with few exceptions, com- 

 mon to all plants, other brilliant coloring-matters occur in some 

 of these lower plant forms which are peculiar to whole fami- 

 lies and correlated with special physiological functions. 



The general distribution of chlorophyll, with few exceptions, 

 in all plant groups is only second to the proteid compounds; 

 however, the color of this compound is not the same tint in all 

 plants, and the evergreens and many other plants when com- 



1 Plate I illustrates this principle for the three horizontal planes, which is 

 also applicable to the orders. 



