GROWTH, IRRITABILITY AND MOVEMENT 147 



shoot : it controls the growth of the stem, induces leaf development, 

 and promotes internal differentiation. To some extent the light 

 may operate through its effect on the growth-regulating substances 

 of the shoot (see p. 148). In addition the reduced elongation of the 

 stem may be associated with a general decrease in cell-turgor 

 which results from the occurrence of more rapid transpiration in 

 the presence of light. Experiment shows that the blue and violet 

 constituents of white light are the most effective in this connection : 

 for instance, a plant grown in red light is almost as markedly etiolated 

 as one grown in darkness. 



Although growth in the usual sense of the term is obtained in dark- 

 ness in the examples quoted above, yet the plant as a whole is steadily 

 decreasing in dry weight, since respiration is not counter-balanced 

 by assimilation of fresh material. The dry weight of those parts 

 of the etiolated plant that are actually developing — the root and 

 shoot — would, however, show some increase, since food will be 

 drafted into them from the storage tissue. 



It is the want of sufficient light for normal development that makes 

 plants in crowded greenhouses and dwelling rooms grow " leggy," 

 with unduly lengthened stems and leaf-stalks. Crowding of field 

 crops has a like effect, and often leads to the " laying " of corn under 

 heavy wind and rain before harvest, the plants being top heavy 

 and their stems weak. The apparent stimulating effect of darkness 

 on stem elongation has the advantage to the plant that the young 

 shoots of seeds or other organs sprouting underground grow rapidly 

 up through the soil, and are able to enter on photosynthesis earlier 

 than would otherwise be the case. In consequence of the controlling 

 effect of light, the rate of elongation of shoots is more rapid during 

 the night than in the daytime, provided the temperature is not too 

 low : while the short stature and rosette form of alpine plants is 

 partly due to the high light-intensity to which such plants are exposed. 



Temperature. 



It has previously been remarked that vital activity in general, 

 which includes growth, is only possible within a certain range of 

 temperature, which for many plants is approximately 0° to 45 C. ; 

 though there is considerable variation in this respect. Species from 

 temperate regions have lower temperature requirements for their 

 growth than those from the tropics. Alpine and arctic plants have 

 still lower requirements. It is a common fact of experience that 

 in temperate climates a rise of temperature from the normal accelerates 



