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 

 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 Raniincubis aqiiatilis,- Cabomba, or 

 water marigold (Bidens Beckii), found growing in shallow 



g5. — Water crowfoot (^Ranunculus) 

 with two forms of leaves. 



Fig. 96.— Transverse sections of leaves of water marigold (_Bidens Beckif). 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- 



