170 PHYSIOLOGICAL BOTANY. PART If. 



or flowering ; for although it is evident that the re- 

 gular return of the seasons stimulates all plants to a 

 periodic execution of these functions, and although 

 the great majority of individuals of the same species 

 and under the same circumstances perform them at 

 nearly the same time, yet it often happens that some 

 individuals are considerably retarded or accelerated in 

 these respects. But further than this, the functions 

 themselves, independently of the action of any external 

 stimuli, appear to have a natural inherent tendency to 

 periodic returns of activity and repose. Thus in the 

 animal kingdom, the return of night and day are met 

 by a desire to sleep and to be awake ; and although 

 these desires may be so modified in different individuals 

 that some require less sleep than others, there are cer- 

 tain limits beyond which it is not safe to carry any 

 unnatural attempts to live without it. Now as in 

 these cases we do not attribute the periodic desire 

 to sleep to the regular return of night, but to the cha- 

 racter of the function itself ; so in the case of the 

 diurnal opening and closing of flowers, the phenomenon 

 must primarily be ascribed to some inherent quality 

 in the plant, assisted indeed by the stated returns of 

 the stimuli to which it is subject. 



(152.) Functions of Vegetation. Whether we con- 

 sider life in the vegetable kingdom as possessing more 

 than one property or not, the various operations which 

 result from its action, upon and through the instru- 

 mentality of the several organs of which plants consist, 

 are termed " functions of vegetation." Although there 

 are a multiplicity of operations carried on in different 

 parts of the vegetable structure, they may all be con- 

 sidered subordinate to one or other of the two general 

 functions of nutrition and reproduction. By the former 

 the life of each individual is preserved, and by the latter 

 the continuance of the species is secured. 



(153.) Stimulants to Vegetation. Life, in order 

 to act through the instrumentality of the vegetable 

 structure, requires to be stimulated by the influence of 



