426 MANUAL OF BOTANY 



osiuotically from the soil by the root -hairs leads to a great 

 tiu'gescence of the tissue of the cortex of the root, which tur- 

 gescence exerts considerable pressure on the sides of the vessels 

 and tracheids of the x3derQ of the stele. By this means water 

 with various salts and other constituents in extremely small 

 quantity, is forced into the fibro -vascular tissue. The process 

 is not a purely physical one of filtration under pressure, but is 

 regulated to some extent by the protoplasm of the most internal 

 cortical cells. When these are distended to their greatest 

 capacity the protoplasm appears to be stimulated, perhaps by 

 the very distension, and in consequence to allow water to 

 transude through its substance. This mode of response to 

 stimulation is not infrequent in vegetable tissues ; indeed, it 

 appears to correspond to the response of a muscle to stimulation 

 by the process of contraction. The protoplasm thus relieves 

 itself of the over- distension, and we get an intermittent pumping 

 action set up, which has a certain rhythm. By it large quanti- 

 ties of liquid are continually being forced into the axial stele. 



This water, under particular conditions, may accumulate in 

 the vessels, and its presence can then very readily be demon- 

 strated and the force of the root-pressure measured. If a vine 

 stem be cut through in the early spring, before its leaves have 

 unfolded,' a continuous escape of water takes place from the cut 

 surface, and the vine is said to bleed. This phenomenon is not 

 peculiar to the vine, but is shared in by all other terrestrial 

 plants. In many herbaceous forms this forcing out of water by 

 root-pressure may be seen without cutting the plant at all. 

 Many grasses and other herbaceous plants show in the early 

 morning a certain exudation of water from the tips of their 

 leaves, which is due to the same over-turgescence. 



To measure the root-pressure in a plant the apparatus shown 

 in fig. 1184 may be used. It consists of a T-piece of glass tubing 

 which is fastened by indiarubber rings to the top of a cut stem, 

 such as that of Helianthus. To the side arm of the tube a bent 

 tube of capillary bore is attached by a tightly fitting cork, and 

 the T-piece is filled with water. Mercury is poured into the 

 bent tube till it stands at a level a little below the cork. As 

 the root continues to take up water it forces it into the tube R, 

 whence it overflows into the capillar^' tube, causing the mercury 

 in its two limbs to be at unequal levels. By the displacement 

 of the mercury the force of the root-pressure can be esti- 

 mated. 



The root-pressure of various plants has been measured by 



