THE GROWTH OF ROOTS AND STEMS. 155 



about 45, and notice how (1) the main root and (2) the side roots 

 change their direction of growth : how does the shoot behave ? 



(c) What happens if a shoot, laid horizontally, is fixed so that it 

 cannot curve upwards? Lay a few Bean-seedlings on moist sawdust 

 and keep the shoots down with a piece of thick glass, or fix them to a 

 sheet of cork by means of crossed pins, and set them in moist air for 

 about six hours. Then remove the glass or the pins : the shoot will 

 quickly bend upwards. How can you explain this result ? 



(d) Fit a Bean-shoot into a bottle or tube of water, using a bored or 

 split cork and sealing with plasticine, and let the shoot project hori- 

 zontally. Stick a pin or needle into the free end of the shoot and set 

 up beside it a foot-rule ; note the position of the index-pin on the 

 scale. After half an hour (the shoot will have made little or no up- 

 ward movement in that time), turn the bottle round through 180, 

 taking care to keep the free end at the same point on the scale. 

 The shoot soon begins to curve downwards, then it comes to rest, 

 and finally it curves upwards. Now try to account for these 

 results. Remember that there is an interval between (1) the perception 

 of the stimulus and (2) the visible response made by the root. 



186. How does Moisture affect the Direction of 

 Growth? Is the main root bound to grow vertically down- 

 wards in all circumstances, or could it be induced to change 

 its course by some other stimulus ? We know that roots 

 require moisture. Would the main root leave its vertically 

 downward course in order to grow towards a moister part of 

 the soil ? Would this be a useful thing for the root ? 



* (a) Lay a glass jar on its side (a jar with flat sides is best) and put 

 a sponge, soaked with water, in the closed end. Fit the jar with a 

 cork (bored with a hole for ventilation), and to the inner side of the 

 cork fix a Pea or Bean seedling by putting two pins through the coty- 

 ledons. Fix the cork so that the root lies horizontally. Does the root 

 bend and grow towards the wet sponge, the shoot towards the mouth 

 of the jar ? 



(b) Grow seeds hi a box with a bottom of wire gauze, and hang the 

 box up or tilt it by putting a wood block under one end. The roots 

 grow down through the gauze into the dry air, but they soon curve and 

 grow upwards again. Why ? 



(c) Take a tumbler or jar about a quarter filled with water, tie over 

 it some muslin, and on it place some damp sawdust with several 

 soaked seeds, then invert a larger tumbler over the whole. The roots 

 on emerging from the sawdust do not turn back, but grow down in the 

 damp air. Compare with (b). 



(d) In the middle of a box of dry soil or sawdust place a flower-pot, 

 first plugging the hole in it by a cork. Place some soaked seeds in the 



