ought to the point where it
will afford definite knowledge of either the physiological problems
involved or of their practical applications in questions of soil
productivity, etc.

CHAPTER XVIII

ADAPTATIONS

Most of the discussions which have been presented in the preceding chapters
have dealt with the types of compounds, the kinds of reactions, and the
mechanism for the control of these, which are exhibited by plants under
their normal conditions for development. The results of the evolutionary
process have produced in the different species of plants certain fixed
habits of growth and metabolism. So definitely fixed are these that in each
particular species of plants each individual differs from other
individuals, which are of the same age and have had the same nutritional
advantages and environmental opportunities for growth, by scarcely
perceptible variations, if at all. Indeed, this fixed habit of development
makes possible the classification of plants into genera, species, etc.
While _different species_ of plants, given the same conditions of nutrition
and environment, produce organs of the widest conceivable variety in form,
color, and function; within the _same species_, the form and size of
leaves, the position and branching of the stem, the color, size, and shape
of the flower, the coloration and markings of the fruit, etc., are
relatively constant and subject to only very slight modifications.

It is unnecessary to say that the mechanism, or the impulses, which govern
the morphological characters of the tissues which any given species of
plants will elaborate out of the crude food material which it receives from
the soil and atmosphere, are wholly unknown to science. It is the commonly
accepted assumption that the fixed habit of growth of the species is
transmitted from generation to generation through the chromosomes of the
germ cells. But the nature of the elements, or substances, which may be
present in the chromosomes, which influence the character of the organs
which will develop months later, after the plant which grows from the germ
cell has gone through its various stages of vegetative growth, is still
altogether unknown. There can be no question, however, that some influence
produces a fixity of habit of growth and development which is almost
inevitable in its operation.

But while this unvarying habit of growth is one of the