666 



PLANT MOVEMENTS 



of the influence of gravity. The stem tips of twiners are usually long and 

 slender and devoid of leaves. Their mechanical tissues are not well developed 

 so that the tip of the stem usually droops over into a more or less horizontal 



L£Ar IN ROLLED CONDITION 



,;Wv\nnnnrMlYWliTinn;i^^ 



LEAF IN EXPANDED CONDITrON 



bull/form ctlli 



Fig. 148. {A) outline drawing of 



blue grass {Poa pratensis) leaf in Fig. 149. {A) outline drawing of sand 



folded condition, {B) in an expanded reed (A mmophila) leaf in rolled condition, 



condition, (C) detailed drawing of (B) in an expanded condition, (C) detailed 



mid-portion of the leaf showing the drawing of mid-portion of the leaf showing 



"bulliform cells," changes in the tur- a few of the "bulliform cells," changes in 



gor of which determine the folding the turgor of which are responsible for the 



and opening of the leaf. rolling and opening of the leaf. 



position. More rapid growth on the lower and outermost sides of the stem 

 results in the upward swinging movements of the tip. This growth is as- 

 sociated with a twisting of the stem which follows as a result of the unequal 

 growth rates so that different segments of the tip become successively placed 



