2 EVOLUTION OF PLANTS 



microtome, improved methods of staining, etc., as well 

 as the extension of explorations to the remoter parts of 

 the world, have all contributed to these advances in 

 knowledge, and have been fruitful sources for new 

 materials. The brilliant results already attained make 

 it reasonable to hope that others of equal importance 

 are yet to follow. 



Very much remains to be done, and any conclusions 

 based upon the data now at hand must be subject to 

 change as new facts are brought forward ; nevertheless 

 enough is at present known to warrant an attempt, at 

 least, at an arrangement of the larger groups of plants, 

 showing their mutual affinities. Some of these groups, 

 however, like the Diatoms, stand very much by them- 

 selves, and it must be admitted that their relationships 

 with other plants are extremely problematical. 



In spite of the almost infinite diversity of structure 

 shown by plants and animals, it is found that, only a 

 very small number of the seventy or more chemical 

 elements known at present, enter into their composition. 

 Carbon, oxygen, hydrogen, nitrogen, and probably sul- 

 phur are always present, and usually at least, potassium, 

 phosphorus, calcium, magnesium, and iron. In addition 

 to these are a number of elements usually present, but 

 apparently not essential for the manifestation of life. 

 These elements are combined into extraordinarily com- 

 plex substances whose exact analysis often baffles the 

 chemist, owing to their excessive instability. Inasmuch 

 as none of the elements found in living matter are 

 peculiar to it, but are also found in the so-called " in- 

 organic " substances, and since all living bodies are 

 directly or indirectly dependent upon the latter for 



