70 PLAN! PHYSIOLOGY 



lus; light is probably not the direct source of the energy- 

 used. The energy used may be wholly of chemical origin, 

 the elaboration of nitrogenous foods taking place by purely 

 chemosynthetic processes; or it may be furnished by the 

 physiological oxidation (respiration) taking place in the 

 cells in which the compounds are being formed. 



The nitrogenous foods are found in plants either in soluble 

 or insoluble form. The soluble compounds are the only ones 

 immediately useful. They can be transferred from cell to 

 cell, they can be directly acted upon by the various chemical 

 influences brought to bear upon them by the living cells, 

 they can be assimilated — converted into substances like 

 those composing the living substance — and when so elabo- 

 rated and assimilated, they become a part of the living 

 protoplasm. In soluble form they occur mainly as amides, 

 of which asparagin is the most abundant and probably the 

 most important. The amides are found in greatest abun- 

 dance in parts where the formation and growth of new 

 cells are most rapidly taking place, that is, in those 

 parts which demand most food of non-nitrogenous and 

 nitrogenous sorts for construction of new parts (cell- walls 

 and protoplasm) and for energy. From analogy with the 

 known behavior of the carbohydrates, and from experiment, 

 it may be concluded that the amides are not elaborated in 

 growing parts, but rather are transferred to them from 

 parts where they have been elaborated from sugar and in- 

 organic nitrogen compounds, or from parts where the 

 already elaborated nitrogen compounds have been stored. 

 These compounds are stored, in most cases, in insoluble 

 forms (e. g: the proteids), corresponding in this physical 

 quality with the starches and oils, and associated with them 

 in the cells of the storing-tissues,— in the pith, medullary 

 rays, the cortical and pith parenchyma of roots, tubers, and 

 other underground parts, in seeds, and in spores. 



The complex nitrogen compounds are very unstable and 

 are doubtless subject in the plant to more or less constant 

 destruction and reconstruction. The destruction of complex 

 nitrogenous compounds does not result in the higher plants 

 in the excretion of waste substances, as in the animals. 



