THE GROWTH OF BOOTS AND STEMS. 153 



curved so as to be directed downwards ? Have you noticed 

 how, if a seed is laid on firm soil, root-hairs grow out and fix 

 the root to the soil, giving it a "purchase," so that it can 

 push its tip down with greater force? Have you noticed 

 how the lighter kinds of seeds are often raised up or turned 

 over by the struggles of the root to bore into the soil ? 

 Can the downward pushing power of the root be measured, 

 say in a Broad Bean seedling ? 



* (a) You have probably used mercury in various experiments, and 

 know that it is a very heavy liquid (13 times heavier than water). 

 Fix a seedling (Bean, Pea, etc. , should be tried) to the side of a small 

 dish containing mercury with a layer of water above it, and see whether 

 the root will grow down into the mercury. The seeds may be pinned 

 to a cork which is securely fixed to the rim of the dish (e.g. a saucer) 

 by making a slit in it and jamming it tightly on the rim ; each seed 

 should of course be fixed by two pins. 



(6) One method of roughly measuring the pushing power of the root 

 is to make it press a cork, or a chain of corks, downwards into water. 

 Cut a slit in a cork so that it can be firmly fixed to the rim of a tall, 

 narrow jar or a large test-tube, and to this cork pin a Bean seedling 

 (with a root about 2 ins. long) so that its root projects into the mouth 

 of the tube. Then place in the tube a number of corks, one above 

 another, of such size that they can move freely inside the tube ; the 

 row of corks should be about half as long as the tube. In the upper- 

 most cork make a cavity for the root to press downwards against, and 

 pour some water into the tube so that the root fits into this cavity. 

 Place a vertical strip of paper on the tube, on which to mark the 

 position of the lowest cork from day to day, and put some wet cotton- 

 wool or blotting-paper over the seedling to keep it moist. 



Each day pour a little more water into the tube (which should be 

 fixed in the vertical position), and thus increase the resistance against 

 which the root has to push downwards. When the root seems unable 

 to push the chain of corks any farther down, mark the position of the 

 bottom of the lowest cork, then take the seedling out arid find what 

 weight (e.g. of shot in a test- tube) is required to depress the corks 

 to the mark. By finding the diameter, and from this the area, of the 

 root just above the tip, you can calculate the pressure of the down- 

 growing root in grammes or ounces per square centimetre or square 

 inch. 



(c) Fix a young Bean-seedling so that its root grows in a small tube 

 filled with moist soil or sawdust, and place this tube within a larger 

 one containing a spring. The root grows downwards with a force equal 

 to over 300 grammes (about 11 oz.) ; measure the diameter of the root 

 and calculate the force it exerts per square centimetre or square 

 inch. 



