DOWNWARD GROWTH OF THE ROOT 



35 



E. Find an average between the wilting temperature and the reviving 

 temperature. For what does this average stand ? Repeat the experi- 

 ment with oat seedlings. 



Reference. Pfeffer-Ewart, 31, I. 



EXPERIMENT XVII 



Do all parts of the root of the Windsor bean seedling bend downward alike ? 

 Fasten some sprouting Windsor beans with roots about an inch in length to 

 the edges of a thick disk of pine wood or other soft wood in a soup plate partly 

 full of water and cover them with a low bell jar. 



Steel pins run through the cotyledons, as in Fig. 2, will hold the beans in 

 place. Mark the roots, as in Exp. XIII, to see in what region the bending 

 occurs ; that is, whether in the older part or by the addition of. new material 

 at the tip. When the roots have begun to point downward strongly, turn 

 most of the beans upside down and pin them in the reversed position. If 

 you choose, after a few days reverse them again. Make sketches of the vari- 

 ous forms that the roots assume and discuss these. 



References. Detmer-Moor, 9 ; PfeSer-Ewart, 31, III. 



EXPERIMENT XVIH 



Does the Windsor bean root tip press downward with a force greatef than its 

 own weight? Arrange a sprouted bean as shown in Fig. 2,i selecting one 

 that has a root about twice 

 as long as the diameter of 

 the bean and that has 

 grown out horizontally, 

 having been sprouted on a 

 sheet of wet blotting paper. 

 The bean is pinned to a 

 cork that is fastened with 

 beeswax and rosin mixture 

 to the side of a little trough 

 or pan of glass or glazed 

 earthenware. The pan is 

 filled half an inch or more 

 with perfectly clean mer- 

 cury, and on top of the mercury is a layer of water. The whole is closely 

 covered by a large tumbler or a bell glass. Allow the apparatus to stand 

 until the root has forced its way down into the mercury. Then run a slender 



1 Or see Ganong, 10. 



Fig. 2. A sprouting Windsor bean pushing its 

 root tip into mercury 



s, seed; r, root; w, layer of water; m, mercury 

 After Sachs 



