1 84 



PLANT LIFE. 



to bring about permanently unequal growth in a certain part, 

 or to cause one part to grow more or less rapidly for a time 

 than another. Such variations in growth produce curvatures 

 in the parts concerned and move members connected with 

 them. They are therefore discussed in the chapter on Move- 

 ments. Those conditions which act more generally and 

 uniformly upon a large number of plants have a tonic effect 

 and serve to determine the form and mode of development of 

 members. 



262. Light.- -The tonic effect of light is different upon 

 different plants and even different members of the same plant. 



FIG. 185. Part of the transverse sections of the stem of rye. A, from a plant grown 

 fully exposed to light; , from a "laid" plant imperfectly exposed to light, a, 

 epidermis ; b, c, mechanical tissues ; d, thin-walled tissues. Highly magnified. After 

 Koch. 



In general light retards growth in length. Stems grown in 

 darkness usually become excessively elongated. Those 

 which under normal illumination have internodes very 



