Stimulation and Correlation. 



123 



would soon reach the bottom of the sawdust. After the 

 preparation has been made, set the box in a position where 

 its lower side will be in- 

 clined 45 degrees from the 

 horizontal, and follow be- 

 havior of the roots. If 

 these organs are directed 

 by gravity alone, they will 

 emerge from the lower side 

 of the box, into the air, de- 

 pending vertically. If cur- 

 vatures OCCUr, tO what force Fig. 95. Water crowfoot (Ranunculus) 



or agency are they due? Care 



must be taken, in this demonstration, that the roots are not 



subjected to the action of strong illumination. 



135. Influence of water on form of leaves. Collect 

 a number of plants of Ranunculus aquafilis, Cabomba, or 

 water marigold (Bidens JBeckii), found growing in shallow 



A B 



Fig. 96. Transverse sections of leaves of water marigold {Bidens Beckii~). A^ 

 aerial leaf ; B, submerged leaf. After Pieters. 



ponds or along the margins of streams. Note the difference 

 between the form of the leaves which arise from the sub- 

 merged and from the aerial portions of the stems. Cut sec- 



