252 PLANT PHYSIOLOGY 



organ affects all the other parts of the plant. Injury or 

 loss of a part is always followed in healthy plants by the 

 replacement of the part, either by new tissues, or by an- 

 other part assuming the work of that injured or lost. Thus, 

 if the tip of a pine or other perennial with excurrent stem j 

 is injured, the lateral branches change their direction of 

 growth, and finally the strongest and most rapidly growing 

 of these assumes the direction and functions of the main 

 stem. "Cutting back" stem or root is followed by copious 

 branching. When, as in floating specimens of Marsilia, etc., 

 enough water is absorbed by other parts, the roots soon 

 cease to grow and may even finally disappear. 



We must conclude, then, that the plant is sensitive as a 

 whole because of the sensitiveness of its parts, that the 

 condition of one part affects all the other parts, that cor- 

 relation is a necessity if the organism is to act as an in- 

 dividual or even as an association of cooperating members. 



Besides the forces which, through analysis, we have been 

 able to recognize distinctly, there are complex influences 

 consisting of forces and influences which have so far eluded 

 analysis. The analytical method has not yet exhausted the 

 subject ; more detailed physical and chemical knowledge will 

 come presently. Meantime it is more or less the fashion, 

 under the name of ecology, to view things in the large way, 

 and by feeling rather than by trt application of exact 

 physiological methods, to reach conclusions regarding the 

 effects of environment and of association. The trees of a 

 given species, presenting one appearance when they grow as 

 members of a thick forest, are very different when growing 

 separately in the open. These differences in appearance are 

 due in great part doubtless to differences in the amounts of 

 light and food, of mechanical strain, and of room, but these 

 are not all, nor do we know the relative importance of the 

 separate influences. We do not know why small plants of 

 characteristic species are the regular associates of certain 

 kinds of trees.* Such phenomena show that plants are 



* For example, see Hock, F. Begleitpflanzen der Kiefer in Norddeutsch- 

 land. Ber. d. Deutech. Bot. Ges., Bd. XI., 1893; Coulter, Plant Relations. 

 New York, 1899, etc. 



