126 The Living Plant 



the sterility of some soils, which are therefore "poisoned" rather 

 than "exhausted." The composition of these substances is not 

 known, except that they are complicated and perhaps nitrog- 

 enous, in which case they may be found to belong with this 

 group of the alkaloids. 



The reader will recall that the active properties of the alka- 

 loids were somewhat foreshadowed in the nitrogenous glucosides, 

 and later he will also make acquaintance with remarkably active 

 properties of another kind which characterize not only the pro- 

 teins entering into living protoplasm, but also the enzymes, with 

 their very striking chemical powers. The common feature which 

 distinguishes all of these substances in contrast with the more 

 passive groups is the possession of nitrogen, which seems there- 

 fore to be associated with the most active properties in plant 

 substances. This fact is sufficiently curious in face of the chemical 

 inertness of nitrogen, and one can fancy this element as reluctant 

 to enter into combinations, restless, so to speak, while in them, 

 and making disturbance in its efforts to escape to its original 

 freedom. 



Class VI. The Flesh-Formers, or Proteins 



These are the most important substances made by plants, 

 entering as they do into the composition of living protoplasm. 

 They are more familiar in animals than in plants, for flesh is 

 made up of them; but they are distributed throughout the living 

 parts of all plants, either in the active protoplasm or stored as 

 reserve food, especially in seeds. They are vast in number, elab- 

 orate in composition, and only imperfectly known. Chemically 

 they are distinguished from all of the preceding groups by con- 

 taining not only the elements of the latter, but also sulphur, 

 while some of them possess phosphorus too, so that their com- 

 position may thus be expressed CnHnOnNnSj,(P„). Some of their 

 molecules are of very great complexity; thus, there is an albumin 

 with the formula C720H1134N218O24SS5; and there are other proteins 



