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 
