Mar. 1901.] BOTANICAL SOCIETY OF EDINBURGH 61 



The first can not be substituted by others ; the second 

 and third may, but only within certain limits, for although 

 the substituted element may suit to set agoing the 

 necessary assimilatory changes, yet the secondary products, 

 or secreta of these changes, may be such as to interfere to 

 such an extent as to render the molecule unable to com- 

 plete its cycle. 



To the botanist these remarks are suggestive in reference 

 to the inability of sodium, for example, to replace potassium 

 as a plant food. Again, although potassium is not an 

 element entering into the constitution of living matter, its 

 presence is necessary for perfect plant growth. It would 

 come into types 2 or 3, i.e. as temporarily entering into 

 the molecule so as to facilitate suljsequeut reaction, or, 

 although not entering into the molecule even temporarily, 

 it may act by its affinity to produce important chemical 

 reactions. Similar considerations suggest themselves in 

 reference to the necessity of iron for the proper formation 

 of chlorophyll. 



It will be noted in the discussion of starch formation 

 that the significance of the terms assimilation, disassimila- 

 tion, and secretion is here different from that usually held 

 by botanists. Starch, usually regarded as a product of 

 assimilation, is here classified as a secretion, derived and 

 thrown off from the biomolecules at a particular stage of 

 the assimilatory reactions. 



The secretory products fall into two main groups — (1) 

 Excretory products : these are never assimilated by the 

 biomolecule, e.g. the oxygen evolved during the decom- 

 position of carbon dioxide by the chloroplasts. (2) 

 DisassimikUory products : given off by the biomolecules in 

 the course of their assimilatory reactions, e.g. the carbon 

 dioxide given off as the end result of respiration. 



Attention is then directed to the close relation existing 

 between the food substance, the living substance, and the 

 products of secretion. Disassimilation and secretion are 

 concurrent phenomena of assimilation. There is difficulty 

 in distinguishing between the products of the two processes, 

 and the nature of these products depends not only on the 

 structure of the biomolecules, but also on the chemical 

 composition of the foods. In this connection the varying 



