ROOT-PRESSURE 



213 



the leaf-teeth of many plants (Buttercups, Poppies, Violet, 

 Strawberry, Gooseberry and Currant trees. Foxglove, Daisy) ; 

 from the leaf-tips of others (Grasses, Stitchwort) ; from 

 spots on the leaf-margin (garden " Nasturtium ") ; or from the 

 upper faces of the leaves. These drops appear especially when 

 transpiration is slow, though absorption by the root is rapid. 

 Consequently, if we place a potted plant under a bell-jar and 

 warm the roots, we can easily induce the excretion of 

 liquid water from the leaves. In most cases the water will not 

 ooze from the leaves if the leaves or branches are removed 

 from the plant, even if their ends are dipping in water. If, 

 however, we force water up the cut end of the stem, by means 

 of a column of mercury, as shown in fig. 244, water will more 

 or less rapidly come from the leaves. This suggests that the 

 root in some way pushes the water up the stem and forces it 

 out of the leaves. But in some cases the water will appear 

 even if the leaves are separated from the roots, as in the 

 Fuchsia, just as sugar-solution will pour out of the nectaries of 

 cut flowers. p. 



ROOT-PRESSURE. 



If we cut across the stem of a Vine in early 

 spring, the cut surface of the stump attached to 

 the root will "bleed": in other words, a solution 

 is forced out of the- cut surface. By fixing a long 

 vertical glass tube to the stem, as in fig. 245, we 

 see that the root is able to push the water high up 

 the tube. The power which the root possesses of 

 forcing water upwards is termed root-pressure. If 

 we warm the roots the root -pressure is rapidly 

 increased ; cooling the roots has the opposite effect, 

 so that the column of water remains stationary or ^ 

 rises slowly. Thus root-pressure depends on the 

 rate at which the root is absorbing water. If the 

 leaves of a plant are transpiring rapidly and we 

 cut the stem across, we shall see no "bleeding," 

 there is at first no sign of root-pressure, though in 

 many plants it will appear after a time if the roots be kept 

 warm and transpiration be arrested.* 



* This absence of root-pressure in rapidly transpiring shoots does not 

 imply that the force which causes root-pressure has stopped. 



Fig. 245. 



