158 THE BIOLOGY OF FLOWERING PLANTS 



limit of assimilation is extraordinarily low — down to 

 — 16° C. air temperature ; but at low temperatures and light 

 intensities, though assimilation takes place, no starch is 

 formed. If a plant is placed in a high temperature and the 

 assimilation in different light intensities is determined, it 

 is found that the rate of assimilation increases to a point 

 at which starch makes its appearance ; thereafter a decrease 

 occurs, followed at still higher light intensities by a second 

 increase. The same thing takes place at high light intensities 

 when temperature is the limiting factor and is gradually 

 increased. A comparison of sun and shade plants of the 

 same species showed that for the latter a time factor set in 

 with the temperature as low as 15° C, and for the former 

 at 31° C. For light a time factor set in at the very low 

 intensity of one-twentieth sunlight for shade plants. We 

 may note here that Matthaei (1904) found that assimilation 

 by the cherry laurel decreased markedly in April. As 

 Lewis and Tuttle (1920) have shown for a number of 

 evergreens that sugar content of the leaves falls, and starch 

 is formed in spring, it seems that Matthaei 's result is 

 analogous to that of Henrici for alpine plants. 



Tropical Plants.— Mention may here be made of the 

 work of McLean (1920) on the assimilation of the coconut 

 palm in the Philippines. The rate rises rapidly with rising 

 light and temperature in the morning, reaching a maximum 

 about 9 a.m. It falls off somewhat till about 2 p.m., rises 

 to a second and lower maximum about 5 p.m., and falls 

 rapidly to zero between 6 and 7 p.m. The rapid morning 

 and evening changes correspond to the short twilight of the 

 tropics. The cause of the mid-day fall was not elucidated. 

 McLean suggests that it is due to too intense insolation ; 

 it seems more likely that it is connected with transpiration 

 and a possible closure of the stomata, the condition of which 

 should be examined. Giltay (1898) found that the pro- 

 duction of dry matter by plants in the tropics was not 

 greater than under favourable conditions in Europe. This 

 is explained by Blackman's results ; but as the conditions, 

 especially of temperature, may be favourable for much 



