HOPKINS: BIOCUMATIC LAW 37 



It appears that, in general, all organisms that are adapted to 

 a given environmental influence respond to such influence in like 

 manner. Therefore single species or groups of species of animals 

 and plants can be utilized to interpret and measure the char- 

 acter and intensity of the controlling influences of a place or 

 region. As sources of evidence and facts for the study of the 

 broader phases of the problem of responses of organisms in gen- 

 eral to the controlling influences of the local environment and 

 regional climate, it has been found that plants, and especially 

 trees and hardy shrub species, are the most convenient and 

 reUable. 



HOW THE PI.ANT RECORDS AND MEASURES THE INTENSITY OF 

 THE CONTROIvUNG INFEUENCES 



Trees and shrubs record the intensity of the influences which 

 affect their life processes by means of more or less distinct events in 

 their periods of activity and rest. In the humid climate of the 

 north temperate zones the beginning of the period of activity is 

 simultaneous with the advent of the spring season, in the South in 

 February, and progressively later northward into June. This 

 response is manifested by the swelling and opening of the winter 

 leaf and flower buds, followed by successive events during the 

 seasons of maximum and minimum activities such as the unfold- 

 ing of the leaves, opening of the flowers, first leaves full grown, 

 full foliage, winter buds forming, fruit forming, ripening or fall- 

 ing, autumnal coloring of the foliage, etc. Each event in each 

 individual of a species marks both a stage in the development of 

 the life processes and the advance of the season as related to the 

 controlling influences of the locality or region. 



Variations in the date of a periodical event from a given norm 

 or constant are a measure, in terms of time, of the intensity of 

 the controlling influences and forces as related (a) to geographical 

 position, (6) to the season, (r) to the inherent tendency of species 

 under the same external influences to vary towards early and 

 late individual responses, and (d) to early and late responses of 

 individuals of the same variety under varying local influences. 

 The variation from a constant in the date of an event also mea- 

 sures the intensity of the controlling influences in terms of dis- 



