I30 



PHYSIOLOGY. 



flower plant in the experiment detailed aliove. The day position of tlie 

 leaves on the other han<l, which is more or less horizontal, is induced because 

 of their irritability under the influence of light, the inherent d(_iwnward or 

 epinastic growth is overcome for the time. Then at nightfall or in darkness, 

 the stimulus of light being removed, the leaves assume the position induced 

 by the direction of growth. 



266. In tin- ease of the cotyledons of some plants it would seem that the 

 growth was li}'ponastic e%'en alter they have o]ii.ned. The ilay position ot 



Fig, 122. 



.Squash seedling. Positinn of cotyledons ii. 

 the d.lrli. 



les.^ hori/onl.il, ,is shown in hg. 

 y covering with ,i tight box. the 



Fig. 121. 



Squash seedling. Position of cotyledon^ 

 light. 



the eot\ledons of the ]nnn])kiii is inoie i 



121. At night, or if we darken the phuit 



leaves assume the i:)osition shown in fig. 122. 



While the horizontal jiosition is the general ime which is assumed by 

 plants under the influence of light, their position is dependent to a certain 

 extent on the intensity of the light as well as (jn the incidence of the light 

 rays. .Some plants are so stnmgly lieliotropic that they change their posi- 

 tions all during the day. 



267. Leaves with a fixed diurnal position. — Leaves of some pilants when 

 they arc developed have a lixed diurnal position and are not subject to 



