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 behaves 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 



