ADJUSTMENT TO TEMPERATURE 101 



addition of nitrogen to form proteids. Like nearly all chemical 

 changes in the protoplasm, this process is little understood. 

 Although it occurs regularly in the chloroplasts, it is not con- 

 fined to them, nor indeed to green plants. It is not dependent 

 upon light, and is termed chemosynthesis because the necessary 

 energy is supplied by chemical action. The plant is unable to 

 take nitrogen from the air, but secures it from the soil in the form 

 of ammonia or of nitrates. The latter are combined with glu- 

 cose or with maltose to form amides, diffusible nitrogenous com- 

 pounds convertible into proteids. This action takes place in the 

 presence of potassium, calcium, and magnesium, usually in the 

 form of sulphates and phosphates. While all of these seem neces- 

 sary, potassium alone appears to take part directly, though sul- 

 phur and phosphorus appear ultimately in the protoplasm. The 

 calcium is apparently for the purpose of neutralizing oxalic acid 

 or other injurious compounds arising during the process. The 

 probable reaction has been represented by the following: 



, , potassium . , potassium , , 



glucose +^nitTate =asparagm + oxalate + water + oxygen 



C6H12O6+ 2KNO3 =C4H8N203 + K2C2O4 +2H2O+ O3 



123. Respiration. Light is the original source of energy for 

 all chlorophyll plants, and indirectly for all hysterophytes, with 

 the exception of a few nitrogenous bacteria. The energy of light 

 is, however, available only at certain times. Consequently the 

 plant has been obliged to find a way of storing it, so that it can 

 be used at night as well as in parts of the plant deprived of light. 

 This is brought about in the formation of complex food mate- 

 rials which represent a certain amount of stored or potential 

 energy. In such forms energy may be carried or translocated 

 to various parts of the plant, and stored as starch, oil, or proteid 

 to start the plantlet, so that it can again reach the light and obtain 

 its energy directly. Translocation, storage, and digestion are all 

 as much concerned with the distribution and use of energy 

 as with that of food material that is to be assimilated. This 

 is evident when it is called to mind that a sugar, oil, or pro- 

 teid serves both as a supply of energy and as a constructive 

 material. 



The lil)eration of energy stored in various compounds, which 

 are originally solul^le or rendered so by digestion, takes place 



