105 



wasted on the construction of leaves of normal shape and size 

 and the leaves which are formed are as a rule unusually 

 small or thin, the plant's efforts being mainly devoted to 

 increasing the length of the stem, with the object of enabling 

 it to reach the sunlight where green leaves can once more 

 assimilate carbon from the air. 



Provided that other important factors, such as the amount 

 of available water and the temperature, remain constant, 

 plants, as might be expected, usually grow more at night 

 than in the day. 



The rays of light which have most effect on growth are the 

 so-called ' ' chemical rays ' : which are situated at the blue- 

 violet end of the spectrum. In plants which have developed 

 in a red light, therefore, the retarding effects of light which 

 prevent excessive growth are not seen, and although in such 

 plants chlorophyll is formed, and carbon-assimilation enables 

 them to increase in weight, their growth in other respects 

 resembles that of plants grown in deep shade. 



96. Owing to its remarkable P la nt Move- 



power of irritability the living protoplasm of plants is, as it m 

 were, able to perceive the existence of external factors and 

 to regulate its actions accordingly, a fact which is often made 

 manifest by the obvious movements of plant organs. 



Thus the protoplasm in the roots and stems of the higher 

 plants is sensitive to the force of gravity and, guided by the 

 direction in which this force acts, it is able to direct the growth 

 of the elongating portions of these organs in such a way that 

 they assume the positions best suited for the performance of 

 their functions. Thus if a healthy seedling is placed with its 

 primary shoot and root in a horizontal position, the growing 

 portions of these organs will be found to curve in such a way 

 that the tip of the root and stem respectively point directly 

 downwards towards the centre of the earth and vertically up- 

 wards in a directly contrary direction, growth then being 

 continued in these directions without further curvature. The 

 curvatures to which these movements are due can only take place 

 in those parts of the root and stem which are still growing and 

 they are caused by the unequal growth of the opposite sides of 

 the organ concerned. In the root the upper side grows more 

 rapidly and in the stem the opposite occurs. This phenomenon 

 of movement executed in response to the force of gravity is called 

 geotropism. The root which grows towards the centre of the 

 earth, i.e. in the direction along which the force of gravity is 

 acting, is said to be positively geotropic, while the stem which 



