2:"i I- PHYSIOLOGICAL BOTANY. PART II. 



The peach, apple, and almond are familiar examples. 

 It sometimes happens, when the leaves have been de- 

 stroyed by drought or other causes, that a second crop 

 of flower-buds is developed late in the year ; the trees 

 having sustained a check in their vegetation, similar to 

 what takes place in the winter, break out again as if it 

 were a second spring. 



(24.Q.) Periodic Iiijluriiwx. The periods at which 

 the flowering of plants commences in different years, at 

 a given spot, appear to depend upon the mean distri- 

 bution of temperature per month, rather than upon the 

 mean annual temperature. Since some process or other 

 of the function of nutrition is carried on throughout the 

 year, and even in winter this is not entirely dormant, 

 there may very likely be a critical season, when some 

 defect of moisture, light, or temperature would be fatal 

 to the progress and perfection of a particular process, 

 and retard or completely prevent the flowering of 

 the plant at the proper time. When by a com- 

 bination of circumstances partly dependent on the 

 peculiar constitution of the individual, partly on the 

 character of the species, and partly on external influ- 

 ences the periodic return of a plant's flowering has 

 been fixed within certain limits, to a given month in 

 the year, it requires a certain lapse of time before 

 any alteration in the external circumstances to which 

 it may be subjected, can effect a decided change in this 

 period. Thus, it is observed that plants which are 

 transported from the southern to the northern hemi- 

 sphere, do not immediately accommodate themselves to 

 the opposite condition of the seasons in which they are 

 placed, but for a while continue to show symptoms of 

 flowering, at the same period of the year in which they 

 had been accustomed so to do in their native climate. In 

 some instances they are several years in accomplishing 

 the change, and sometimes even die before they can 

 effect it. The usual limits within which the periodic 

 returns of flowering in each species take place, are 

 always mentioned in the Floras of a given district ; and 



