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E. J. KRAUS 
detailed, in many cases, to afford real clues as to the several forms in which 
an element may be present; a total nitrogen estimation alone, for example, 
is of very little significance in any attempt to determine the nitrogen meta- 
bolism of a plant. And yet, invaluable aid toward an interpretation of the 
problem under discussion can be gained by fitting together the records 
available, and many of the more recent contributions are very helpful. 
Specific quantitative measurements of substances frequently take on an 
entirely new significance when they are no longer considered by themselves 
alone, but rather in connection with other materials present, as ratios. 
When, for example, such suggestive results as have come from various 
experiments designed to determine the nutrient salt requirements of plants 
in various stages of development, under varied conditions of light and 
temperature and moisture, are finally coupled with analyses of the organic 
and inorganic materials in the plants themselves at these different stages of 
development, we shall have begun a genuine approach to the problem of 
metabolism. 
At the outset of any experiment which concerns the functions of growth 
or of reproduction, it is quite as important to determine the condition, or 
better the composition, of the plants which are to serve as the basis for the 
investigation, as it is to know and control the external conditions imposed. 
Many of the apparently discordant results of various experiments are easily 
accounted for and harmonized when the composition of the material used 
as the basis for investigation is taken into account, or when the range of 
effects of any element is considered in connection with the limits imposed 
by other substances present. One has but to think of the effects of nitrogen 
as partially detailed previously, or of sulphur, or phosphorus, or other 
elements which enter into a vast number of organic compounds essential to 
growth, and of how they influence subsequent development when present 
in varying relative quantities. 
At this time it is worth while to consider several points in connection 
with the analyses of tissues of plants and what these may show. The fact 
that plants require or absorb mineral salts in varying ratios, quantities, or 
proportions, means, in other words, that such absorption and utilization 
depend in considerable measure upon the composition of the plant itself, 
and will vary as such composition is varied. Changed or changing condi- 
tion or expression is the external evidence of changed or changing composi- 
tion. The living plant is constantly in a state of becoming adjusted to 
changing surroundings, it is the product of the interaction of all the elements 
of its environment. A change of any one of such elements requires a re- 
adjustment of the entire system unless such a change is at once offset by 
another which in its effects is antagonistic to it. If this is true, the necessity 
of possessing some facts or knowledge concerning the composition and the 
transformation of compounds in the plant in connection with those absorbed 
from the media surrounding it, is absolutely imperative. Anyone who has 
