The Various Substances Made by Plants 133 



This table brings out clearly once more that most fundamental 

 of facts about the physical constitution of living things, that their 

 substance is all derived originally from carbon dioxide and water, 

 with a few minor additions, and is 

 all returned in the end back to the 

 same source, undergoing en route 

 transformations of substance and 

 energy which constitute the princi- 

 pal visible phenomena of life. The 

 organism is made up of a little 

 of those substances temporarily 

 withdrawn from the general cir- 

 culation of nature and interacting FIG. 4i. A ceil, highly magnified, from 



Vigorously with One another Under f Begonia, showing a mass of crys- 



J tals composed of calcic oxalate, lying 



the Stimulus Of external forces, within the cell-cavity around which 



. can be seen the living protoplasm, 



principally the SUn. Organisms (Copied from a wall-chart by L. 



are, as it were, little whirlpools in 



the general circulation of matter and energy. And I cannot for- 

 bear to attempt to illuminate this matter somewhat further by 

 aid of one of my favorite diagrams, which is presented herewith 

 (figure 42). 



There is yet one other group of substances made by plants, 

 very different, however, in kind from those already described. 

 In the tissues of all plants the microscope reveals mineral matters, 

 sometimes in great abundance and crystallized hi very beautiful 

 forms, of which our illustration (figure 41) gives some, though an 

 inadequate idea. A few are probably useless minerals absorbed by 

 the roots along with the useful kinds presently to be noted, but the 

 great majority are by-products of useful chemical reactions. Thus, 

 the commonest of the crystals is oxalate of lime, which is formed 

 from oxalic acid, probably a by-product hi the manufacture of 

 proteins. These crystalline matters are obviously of no use, but 

 are waste materials. In the absence of a regular excretory system 

 such as animals possess, the plant has no resource except to store 



