8 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



on climate in its relation to forest vegetation may sometimes be made 

 on short field trips. Very often the painstaking observer, without 

 extensive apparatus, will discover some fundamental facts which alter 

 the conception of a given problem, and which therefore lead to far 

 more productive efforts by the permanent organizations whicn can 

 study the problem for longer periods. It is only by recognizing this 

 principle of supplemental effort that substantial progress in forest 

 investigations can be made. There should be no attempts to delimit 

 the work of any organization or individual. 



THE SIMPLE PHYSICO-PHYSIOLOGICAL CONCEPT. 



Many ecological problems are less confusing to the beginner and 

 are more likely to be approached by sound methods if, at the outset, a 

 rather definite physical interpretation of life is accepted, for through 

 such a concept is gained an idea as to the probable physical reaction 

 to the environment and the method of measuring the physical con- 

 ditions. 



Thus, to begin with, the living mass of plants (the protoplasmic 

 mass, primarily) may be conceived to be simply a colloidal mass of 

 organic compounds with a peculiar affinity for water. Water is of 

 fundamental importance to its life qualities. To supply the demand 

 for water, the protoplasmic mass must posses a greater affinity for it 

 than the soil or solution from which the water is to be obtained. The 

 struggle for water is, primarily, a contest between the colloids of the 

 plant and the organic and inorganic (clay) colloids of the soil. 



Secondly, it is inevitable that any object possessing water should 

 lose the same by evaporation to the atmosphere until a balance is 

 reached between the vapor pressure of the water-holding mass and 

 that of the atmosphere. Such an equilibrium does not, necessarily, 

 mean death, at least for certain kinds of tissues, but the small supply 

 of water represented by equilibrium with ordinary atmospheric vapor 

 is insufficient to permit photo-synthesis, metabolism, and transport 

 within the plant. For continued functioning, the plant must be able 

 to maintain its water supply above this level. 



The objective of physiological functioning is reproduction, to which 

 growth is only incidental. The object in the existence of any indi- 

 vidual plant is to extract enough phosphorus 1 from the soil so that a 

 peculiar accumulation of matter called a seed may be formed, witli 

 a sufficient affinity for water and a sufficiently close chemical combi- 

 nation to enable this embryonic plant to resist all of the forces of 

 disintegration during a period of dormancy. 



» Phosphorus is mentioned only as an example of the vitally necessary elements obtained from the soil. 



