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THE AMERICAN NATURALIST [Vol. XL V 



vert it into pigment. 2 Another enzyme, or, nnder ap- 

 propriate conditions, the same enzyme that brought 

 about the synthesis of sugar and chromogen into glyco- 

 side, may hydrolyze glucoside into sugar and chromo- 

 gen. It is also known that the rate at which a chemical 

 reaction determined by an enzyme goes forward de- 

 pends on the amount of enzyme present. It is there- 

 fore readily seen that the rate at which a given enzyme 

 is produced in the cell may determine whether or not 

 chromogen shall be available for conversion into pig- 

 ment. It is also highly probable that a principal func- 

 tion of the chromatin of the cell is to produce the 

 enzymes which govern at least the rate of many of the 

 metabolic processes in the cell. 3 



It is certain that environmental conditions during 

 ontogeny determine whether pigment shall be produced 

 in a given tissue, even when the potentiality of pigment 

 production is known to be present. Thus, in seeds that 

 have pigmented coats, pigment may not occur, say in the 

 endosperm. Causes similar to those which determine 

 the particular tissue to be pigmented may also deter- 

 mine what portion of that tissue shall be pigmented. 

 The sugar in the pigment cells of the seed coat is pre- 

 sumablv transported there by osmosis from cells some 

 distance awav. Other materials necessary to the reac- 

 tions may be 'brought from other parts of 'the organism, 

 and some of the products of a reaction the accumula- 

 tion of which might retard the reaction may be trans- 

 ported to other parts of the organism as they are pro- 

 duced. We thus have to deal with an exceedingly com- 

 plex problem, many of the elements of which can not 

 even be conjectured in our present ignorance of cell 

 metabolism. It would therefore be idle to attempt to 

 formulate a definite theorv of the processes involved. 

 A sliffht chancre in the i>,>vi»w>,,l»iliK- nf certnin cell walls. 



