CHAP. VII. EPIRRHEOLOGY. 21 



permitted to operate with greater or less intensity. In 

 all cases, there is that happy mean which can so regu- 

 late the vital force as to produce a healthy and vigorous 

 condition of existence ; whilst every increase or dimi- 

 nution in the stimulus applied, only tends to injure or 

 greatly to modify the individual subjected to its long- 

 continued influence. Physiology might be considered 

 as embracing the investigation only of such phenomena 

 as resulted from the healthy condition of the vital 

 functions ; whilst epirrheology would take further cog- 

 nisance of such as resulted from an unhealthy condi- 

 tion of vegetation. Hence this department would lay 

 the foundations of another branch, termed the " noso- 

 logy " of plants, or that science which treats of their 

 diseases ; and also of the extensive subject of " Bota- 

 nical Geography," which makes inquiry into those 

 causes which limit the distribution of various species 

 to certain spots upon the earth's surface. But in a trea- 

 tise like the present we have not thought it necessary 

 to make any distinction between physiology and epir- 

 rheology, nor are we prepared to allow that such distinc- 

 tion is a very judicious one. In order to understand 

 the effects produced by the vital force, it is necessary to 

 trace its operations under various modifications of the 

 external stimuli by which it is controlled, and even ren- 

 dered capable of acting at all. These inquiries relate to 

 the results of an action and reaction between opposing 

 forces, questions which cannot well be separated with- 

 out greater refinement than the subject seems to require. 

 There are, however, certain phenomena, the discussion of 

 which could not be conveniently introduced under either 

 of the two functions into which the vital properties were 

 arranged. Of these we may select as an example the 

 effects produced by the action of gravity upon growing 

 plants. 



(299.) Direction of Roots and. Stems. That the 



roots and stems of most plants constantly develop in 



opposite directions, is a fact too notorious to need a 



comment ; and any deviation from this general law is 



u 2 



