120 The Living Plant 



the fact that they are compounds of glucose with some one or 

 more definite substances, into which they can again be broken 

 up. Some of them contain nitrogen, as for instance the amyg- 

 dalin above mentioned (its formula is C 20 H 27 O n N), which allies 

 them in some measure with the nitrogen-containing substances 

 next to be considered, especially the alkaloids. 



Class IV. The Nitrogen-Assimilates, or Amides 



These substances, dissolved in the sap of plants and having 

 no particular uses to us, are not commonly known; but they are 

 vastly important nevertheless, inasmuch as they constitute the 

 connecting step between the carbohydrates and the indispensable 

 proteins, soon to be considered. The commonest is asparagin, 

 dissolved in the sap of young asparagus plants, from which it can 

 easily be crystallized out. Its formula, typical of the group, 

 is C 4 H 8 O 3 N 2 , which shows the presence of the nitrogen along with 

 the elements of carbohydrates; and there is no doubt that the 

 ultimate source of the materials is the photosynthetic grape sugar 

 together with nitrogen from compounds absorbed with water by 

 the roots. The amides are not known to perform any special func- 

 tion of their own in the plant, and probably find their significance 

 simply as a necessary chemical step in the formation of proteins. 



The incorporation of nitrogen with the elements of the car- 

 bohydrates is a step of the first biological magnitude, since the 

 nitrogen is the most essential and distinctive additional con- 

 stituent of the most important of all biological substances, 

 living protoplasm. We have already considered, (in Chapter 

 II), the source of the plant's supply of carbon, oxygen, and hydro- 

 gen, and must now turn aside from our main theme to examine 

 the source of the nitrogen supply, a subject all the more important 

 because of the fundamental economic bearings it has. Nitrogen, 

 it should be needless to recall to the reader, is the colorless gas 

 which makes up very nearly four-fifths of the atmosphere; and 

 from such an abundance plants ought apparently to have no 



