1 84 



PLANT r.IFE. 



to bring about permanently unequal growth in a certain part, 

 or to cause one part to grow more or less rapidl)' 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 Mo\'e- 

 ments. Those conditions which act more generall)- and 

 uniformly upon a large number of plants ha\'e a tonic elfect 

 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. 



"ILJUULJLJUU ■"" 

 A 



Fir,. 185. — Part of the transverse sections of tlie stem of ryt. A, frnm a plant grown 

 fully exposed to light; />, from a "laid" plant impcrfectlv exposed to liglit. a^ 

 epidermis ; /', c, mechanical tissues ; d^ thin-walled tissues. lAighly niagnilied. — After 

 Koch. 



In general light retards growth in length. Steins grtnvn in 

 darkness nsually l)eeo]"ne excessi\elv elongated. Those 

 whicli under normal illiinn" nation have intern odes very 



