92 



PLANT SOCIOLOOV 



species to the daily and seasonal march of temperature (c/. McCrea, 

 1924). 



McCrea has made minute investigations of the relation of flowering 

 to temperature in northern England and has expressed the results by- 

 graphs (Fig. 49). 



In most plant communities two or more distinct flowering "aspects" 

 may be recognized. 



In the California chaparral the species are grouped according 

 to time of flowering into five distinct aspects. The first group blooms 

 in January and February with a mean minimum of 5° and a mean 



70" f- 



1. Jan. 



10 15 20 25 



~*' Weeks 



-Temperature 1922 



-Temperature 1923 



30 



~ir to 



21 August 



Species flowered 1922 



— Species flowered 1923 



Fig. 49. — Number of species of plants which had flowered and the temperature 

 curve for the north of England for the first 40 weeks of the years 1922 and 1923. {After 

 McCrea.) 



maximum of 10°C. The second follows in April and May (mean 

 minimum 10°, mean maximum 15°) ; the third in June (mean minimum 

 15°, mean maximum 20°) ; the fourth in July (mean minimum 20°, mean 

 maximum 25°) ; and the fifth in September with a mean minimum of 

 25° and a mean maximum of 30°C. The individual waves of bloom 

 released by increasing heat appear at regular intervals, according to 

 Setchell (1925), separated by a difference of about 5°. Graphic 

 expressions of succession of bloom in various associations of western 

 Switzerland are given by H. Gams (1918). The sociological side of the 

 study of aspects has been discussed on an earlier page (47). 



Isotherms and Boundaries of Vegetation. — The exact relation 

 between the boundaries of vegetational areas and temperature has 

 often been sought. Especial emphasis has been placed upon the 



