MOVEMENT IN PLANTS. 235 



If a plant is laid horizontally for a time, and then removed from 

 the geotropic stimulus by being rotated on a clinostat, whether or 

 not it will make an "after-effect" curvature depends upon the 

 period during which it was horizontal ; determine for various seed- 

 lings this presentation time, or minimum period of horizontality 

 sufficient to produce a reaction in the Bean radicle it is about 

 20 minutes, in the Bean plumule about 7 minutes. But this does 

 not mean that the root or shoot requires so long a period to perceive 

 the stimulus ; experiments with the intermittent clinostat show that 

 perception time is so short as to be practically instantaneous. It 

 would obviously be disadvantageous if the plant were to respond to 

 every momentary stimulus it might receive in nature. 



IV. HYDROTEOPISM. 



313. Hydrotropism. The following experiments de- 

 monstrate the positively hydrotropic curvature of roots 

 towards the moister part of the soil, showing that if the 

 moisture of the soil is not evenly distributed the root will 

 turn aside from its normal downward vertical course. 



(a) Grow seeds of Pea, Sunflower, etc., in wet sawdust 

 in a sieve, or in a box with the bottom replaced by wire 

 gauze, and hang the sieve or box in an oblique position or 

 tilt it by putting a support under one end. The roots 

 grow down through the gauze into the air, but they soon 

 curve and grow upwards again into the wet sawdust this 

 may be repeated several times, so that the roots become 

 threaded through the meshes. 



(b) Take two glass jars or tumblers or beakers. Place 

 some water in A (about a quarter full) ; keep B dry, and 

 in it place some calcium chloride (to keep the air dry). 

 Tie over each a piece of coarse muslin, on the muslin place 

 wet sawdust or sphagnum, with some seeds (Sunflower, 

 Mustard, etc.), and cover all with alarge jar or bell-glass. 

 In A the roots on emerging from the muslin do not grow 

 back into the wet sawdust (as they do in B), but grow 

 down into the moist air. 



(c) Fill with water a porous pot, of the kind used in 

 electric batteries, and securely cork it. Soak a strip of 

 flannel in water, and tie it lengthwise over the pot, putting 



