236 
AMERICAN JOURNAL OF BOTANY 
[Vol. io, 
generic type; and that the crystals of each species of a genus may be 
distinguished from those of another species of the genus. The various 
crystal forms depend, obviously, upon the chemical properties of the 
haemoglobins. More recent work of Reichert shows that specific and 
generic types of starch may be distinguished both by the microscopic 
structure of the grains and by the chemical reactions of the starch. 
Such evidence, when it is all considered, is fragmentary compared with 
the detail of structural differentiation; but it is ample to warrant the 
conclusion that organisms differ in the chemical constitution of their proto¬ 
plasm, and it remains for subsequent analysis to determine the full extent 
of this chemical characterization. 
Ill 
The increase in size of cells or organisms involves that formation of 
new material, which must be largely, if not entirely, synthesized within the 
developing cell. In many cases these synthetic reactions proceed with 
great rapidity, whereas outside the cell such reactions, if they occur, proceed 
at an exceedingly slow rate. The most generally accepted explanation 
attributes to enzyms the rapid rate of these synthetic reactions. Moreover, 
the same enzyms which operate in hydrolytic reactions are evidently equally 
capable of catalyzing the opposite or synthetic reaction. On this basis, 
therefore, it will follow that under favorable conditions those substances 
which a cell is able to digest may also be synthesized by that cell. 
One of the most conspicuous characteristics of enzyms is their high 
degree of specificity. From Pasteur’s classical demonstration of the relation 
of Penicillium glaucum to the tartaric acids we now have a long and familiar 
list of enzyms which react only with their particular chemical compounds 
and fail to react with other compounds. The distribution of enzyms in 
living cells, moreover, exhibits many limitations. This is particularly 
evident among the bacteria, and its utilization constitutes one of the unique 
features of bacterial classification. In the co/i-typhoid group, for example, 
the members exhibit a wide range of activity toward carbohydrates: from 
species which ferment only certain monosaccharides to organic acids, to 
species which ferment many monosaccharides and disaccharides to carbon 
dioxid and water. The division of species of bacteria which liquefy gela¬ 
tine, split certain nitrogen compounds to indol, and coagulate casein, from 
species which do not possess these enzym activities is one of the most fre¬ 
quently observed of bacterial processes. 
The distribution of these enzyms is, moreover, a constant factor. The 
observation that an enzym is produced in reaction to the chemical nature 
of the substratum appears to be true only to a certain degree. An enzym 
already present in a cell may in some cases be produced in increased amounts 
by reaction with the substratum, or enzym action may be suppressed by 
the conditions of the environment; but where an enzym is normally absent 
