CHIEF ENVIRONMENTAL CONDITIONS. 147 



for example, by which cell-walls are thickened or modified, be increased 

 in velocity, this internal change must directly or indirectly alter the 

 concentration and chemical nature of the solutions of the affected 

 tissues. The formation of cork, cuticle, etc., profoundly alters the 

 transpiring power of aerial plant surfaces, and similar modifications 

 in roots must produce a changed absorbing power for solutes as well 

 as for water. 



It is thus emphasized how difficult and arbitrary must be any 

 attempt sharply to distinguish external from internal conditions, and 

 how practically impossible it is at the present time logically to analyze 

 the latter so as to begin to attain quantitative information concerning 

 the various relations that have been roughly and crudely outlined in 

 this chapter. Our reason for submitting this unsatisfactory treatment 

 of the general subject of plant-relations is that the fundamental im- 

 portance of these is not only theoretically but practically very great, 

 and it seems time that a systematic beginning were made in some of 

 the directions suggested by the foregoing incomplete analysis. If our 

 treatment stimulates quantitative and comparative studies of plant 

 environments, so that the present pubUcation may soon be looked 

 upon as useless and quite out of date, our aim will have been realized. 

 The difficulty involved in really scientific studies of plant-relations 

 ought not to be a legitimate reason for their omission and for the 

 continuation of the pioneer sort of qualitative descriptions and teleo- 

 logical interpretations, which appear to belong rather in the realm 

 of "nature-study" and natural mythology than in that of true science. 

 There are, however, already many ecological and agricultural studies 

 on record, wherein the more logical point of view of the more ad- 

 vanced physical sciences is given prominence, and the future of this 

 aspect of biology seems to be assured. 



VIII. EXPERIMENTAL DETERMINATION OF RELATIONS BETWEEN 

 PLANT ACTIVITY AND ENVIRONMENTAL CONDITIONS. 



It is perhaps not out of place here to devote some space to a con- 

 sideration of the general character of the methods which must be 

 employed in the more accurate determination of the relations with 

 which the present chapter has had superficially to deal. As in all 

 such cases, the only possible method of procedure is the experimental, 

 and the experiments must be carried out with all the foresight and 

 logical planning that characterize the work of the modern physical or 

 chemical laboratory. The importance of this line of inquiry can not 

 be overestimated; it is to be regarded as quite indispensable to the 

 scientific advancement not only of ecological knowledge but of that 

 most essential of all human activities, agricultural practice, and its 

 pursuit is surely well worth the time, energy, and money that it 

 would require. 



