THE COMMONER METABOLIC PRODUCTS 453 



organs of the same plant, or even in the same cell. This applies also to the 

 nitrogenous reserve-materials which are present mainly in the form of 

 amides and proteids, although inorganic nitrogenous compounds may 

 sometimes function as reserve food-material. 



Food-materials are often much modified in order to render them suitable 

 for storage: thus starch or oil are often formed when sugar is supplied, 

 and proteids when amides are present. Even where no marked meta- 

 morphosis seems to recur a non-diosmosing compound is probably produced 

 by some slight change or combination with another substance, so as to 

 render further accumulation possible. Similarly the reserve-materials may 

 be mobilized either by slight metamorphoses or by more or less complete 

 disintegration, and during the process of translocation such changes as 

 these may be repeated more than once. Sugar is frequently formed from 

 starch and oil, while proteids may disintegrate into amides, or even into 

 inorganic compounds such as ammonia, phosphoric and sulphuric acids. 



These changes can easily be followed when they occur at different 

 times and in different parts of the plant, but in all cases they involve 

 a certain consumption of energy, and since living organisms always work 

 as economically as possible, it is safe to conclude that whenever the 

 processes of mobilization and translocation involve marked disintegration, 

 the latter is either necessitated by existing circumstances "or is adapted 

 to the end in view. 



The nutrient material is used in a variety of ways, and from it all 

 formative substances required for building purposes are produced after 

 undergoing more or less marked modification, which varies according to 

 the similarity or dissimilarity between the food-materials and those required 

 in constructive metabolism. Food-substances seem almost always to 

 undergo some change, however slight, in the process of assimilation ; 

 as far as our knowledge goes the proteids which form part of the living 

 protoplast differ somewhat from reserve-proteids, and similarly the cell- 

 wall differs slightly in composition from reserve-cellulose. The existence 

 of such differences is, however, not necessarily essential, for since the 

 characters and properties of an organism are determined in every case 

 by the manner in which the different component substances are combined 

 together, it is always possible that one or more of these substances may 

 be directly assimilated by the protoplast without undergoing any chemical 

 change. 



Growth and vital activity are regulated and determined by the general 

 properties of the protoplast, which maintains its specific characters in spite 

 of growth and reproduction. In the progress of development, and under 

 special conditions, there can be no doubt that certain alterations may 

 occur in the physical and chemical constitution of particular parts, and 

 hence substances may perchance be incorporated which previously were 



