228 Influence of Environment on Plants 
and crystalline form. These potentialities can only become operative 
under the influence of external conditions; their stimulation into 
activity depends on the degree of concentration of the various solu- 
tions, on the nature of the particular calcium salt, on the acid or 
alkaline reactions. Broadly speaking, the plant cell behaveg in a 
similar way. The manifestation of each form, which is inherent as 
a potentiality in the specific structure, is ultimately to be referred to 
external conditions. 
An insight into this connection is, however, rendered exceedingly 
difficult, often quite impossible, because the environment never 
directly calls into action the potentialities. Its influence is exerted 
on what we may call the inner world of the organism, the importance 
of which increases with the degree of differentiation. The production 
of form in every plant depends upon processes in the interior of 
the cells, and the nature of these determines which among the possible 
characters is to be brought to light. In no single case are we 
acquainted with the internal process responsible for the production 
of a particular form. All possible factors may play a part, such as 
osmotic pressure, permeability of the protoplasm, the degree of 
concentration of the various chemical substances, etc.; all these 
factors should be included in the category of internal conditions. 
This inner world appears the more hidden from our ken because 
it is always represented by a certain definite state, whether we are 
dealing with a single cell or with a small group of cells. These have 
been produced from pre-existing cells and they in turn from others ; 
the problem is constantly pushed back through a succession of gene- 
rations until it becomes identified with that of the origin of species. 
A way, however, is opened for investigation; experience teaches 
us that this inner world is not a constant factor: on the contrary, 
it appears to be very variable. The dependence of variable internal 
on variable external conditions gives us the key with which research 
may open the door. In the lower plants this dependence is at once 
apparent, each cell is directly subject to external influences. In 
the higher plants with their different organs, these influences were 
transmitted to cells in course of development along exceedingly 
complex lines. In the case of the growing-point of a bud, «which 
is capable of producing a complete plant, direct influences play 
a much less important part than those exerted through other 
organs, particularly through the roots and leaves, which are 
essential in nutrition. These correlations, as we may call them, 
are of the greatest importance as aids to an understanding of form- 
production. When a bud is produced on a particular part of a 
plant, it undergoes definite internal modifications induced by the 
influence of other organs, the activity of which is governed by the 
