110 ENVIRONMENTAL CONDITIONS. 



ing, because simpler and logically more direct, to attend strictly to the 

 various factors as they actually affect the plant, leaving the analysis 

 of the sources of these factors to other studies, perhaps at a later day. 

 We shall here consider only the proximate determining conditions in 

 plant behavior and distribution, merely mentioning a few points bear- 

 ing on the more remote determination of these controlling conditions, 

 which are available from researches in climatology and other fields. 



Since every effect upon the plant must be supposed to be directly 

 traceable to conditions that previously prevailed within and without 

 the organism, and since we make no attempt here to analyze the 

 various physiological processes that constitute the varied plant re- 

 sponses, it has been deemed best in these considerations to regard as 

 an external condition or factor any status of the processes of the ex- 

 ternal world that directly influences changes within the plant. Many 

 logical difficulties arise here, as is usual when any attempt is made to 

 subdivide a continuous series into regions, and the only satisfactory 

 method of procedure is to subdivide the series into arbitrary portions, 

 being sure to define the limits chosen. Thus, for ecological purposes 

 it seems quite as undesirable at the present time to enter into the 

 exceedingly complex physiological considerations involved in a study 

 of the details of plant activities as these are controlled by conditions 

 as it is to take up in detail the more remote causes which bring the 

 various effective conditions into existence. This logical difficulty 

 arises, of course, from the fact that the plant is not an independent 

 system, but is perfectly continuous with the universe about it; the 

 classification, into external and internal, of the conditions determin- 

 ing chemical and physical changes within the plant, is at best but a 

 subjective affair with the mind that classifies. Whether a given con- 

 dition is to be taken as external or internal will always depend largely 

 upon the previous experience and point of view of the observer, upon 

 his internal conditions. For the present needs it seems desirable 

 to base our arbitrary definition of the controlhng factors upon the 

 spatial limits of the plant-body as ordinarily considered. Thus we 

 define as effective external conditions all phases of universal progress 

 outside the plant-body which directly affect the latter in such a 

 manner as to produce alterations in the chemical, physical, and 

 physiological processes that occur within. An example may illustrate 

 this. The passing of a given region of the earth's surface from shadow 

 into sunlight at dawn is not a condition immediately effective upon 

 plant processes, nor is the influx of radiant energy to the surfaces of 

 objects in the vicinity of the organism. The plant-body is first affected 

 in this case when there occurs an increased rate of transmission of light 

 or heat energy through the periphery (or a decreased loss of heat, which 

 amounts to the same thing as an increased income), so that some 

 substance actually a part of the plant-body becomes Hghted or warmed. 



