THIGMO TROPISM. 1 3 1 



it can be made, the apparatus works fairly well, though not so beautifully 

 as when tilled with water. The great value of this arrangement is that 

 it gives a constantly increasing angle away from the geotropic direction, 

 and hence allows the relative strength of the responses to the influences 

 of hydrotropism and gravitation to be determined.* 



This principle of hydrotropism may be demonstrated in several ways, 

 of which the best known is that of Sachs, figured in all books, in which 

 an inclined trough of wire gauze is used. It may also be shown by 

 planting a germinating bean in a small flower-pot to which water is sup- 

 plied by a strip of filter-paper running down between pot and earth and 

 dipping into a water-reservoir outside ; the height of the reservoir must 

 be so adjusted that the soil is moistened slowly from the paper. Flower- 

 pots may be utilized in various ways for demonstrating hydrotropism, 

 advantage being taken of their porosity and their slopes. Another 

 method, recommended by Arthur, is this : Take a funnel and cover its 

 outer surface with filter-paper clipping into a water-vessel which sup- 

 ports it. Soak some large seeds and place them on sand filling the 

 funnel ; their hypocotyls are to project over the edge and they are cov- 

 ered by wet filter-paper. The roots will then follow the wet filter-paper 

 along the slope of the funnel. I have effected the same end very beau- 

 tifully by stoppingthe hole of a flower-pot, filling it with sphagnum and 

 with water, and putting the germinating seeds (covering them with 

 sphagnum) in the same way with hypocotyls pointing over the edge. 

 The roots cling beautifully to the pot. In all these methods it is need- 

 ful to adjust carefully the moisture in the air surrounding the hydrot- 

 ropism-apparatus, which can be done by regulating the height the cov- 

 ering bell-jar is supported above the table; if the air is too dry, the roots 

 wither ; if too wet, they grow straight downwards. 



D. Thigmotropism. 



An influence to which plants are, very naturally, often 

 sensitive, is contact, a property known as Thigmotropism. 



97. What are the principal phenomena of response to contact 

 in tendrils? 



Answer by experiments (Experiment 59) devised by your- 

 selves upon the tendrils of Melothria, Passiflora, Echinocystis, 

 or other sensitive-tendriled plant. 



* Doubtless this arrangement could be used for investigation upon this subject. In 

 some cases the roots after following the cylinder a definite and fairly constant distance 

 turn abruptly downwards, thus giving a fairly constant critical angle,'' which of 

 course depends largely upon the degree of moisture in the chamber. 



