IRRITABLE RESPONSE 



237 



(c) Hydrotropism. 



The tendency of roots to seek wet places is a familiar phe- 

 nomenon of the gardens and a manifestation of hydrotropism. 

 For its study we ask first: 



What are the facts in a typical case of the turning of roots 

 towards moisture? 



This may be tested by so arranging roots growing geotropically 

 downward that they will be deflected, if sensitive, by a moist surface 

 near them. 



EXPERIMENT. Select two clean porous flower-pots of 5-inch or 

 6-inch size; stopper both ends (Fig. 68), and by aid of a round file 



FIG. 68. ARRANGEMENT FOR THE STUDY OF HYDROTROPISM; Xf. 



On the right is a cross-section of the flower-pot about its middle. It may be filled with 

 saturated sphagnum instead of with water. 



worked in a carpenter's brace, bore a small hole near the upper mar- 

 gin; tie lengthwise around, as in the figure, a double strip of cheese- 

 cloth, between which and the pot place a number of soaked seeds; 

 fill the pots with water (or pack them with very wet sphagnum) and 

 suspend them in bell jars, one of which is to be kept with saturated 

 atmosphere, while the other is to be given moderate humidity. Com- 

 pare the positions of the roots in the two, and note approximately the 

 critical angle at which the roots leave the moist surface. 



OTHER METHODS. The arrangement described above serves admirably 

 for this purpose, and permits very effective demonstration, all the better 

 for the great contrast in color between roots and pot. It has the great advan- 



