378 



LECTURE XVI. 



made ineffectual by cutting off the top; or -when, as in the case of the Vine in spring, 

 the transpiring leaf-surfaces are not yet present. 



However, even in the case of plants in full leaf, when the transpiration of the 

 leaves is diminished at a low temperature and in moist air — the roots remaining 

 in a moist warm soil — water may be taken up with great energy, and be pressed 

 forcibly into the le9,ves, to well out in the form of drops, if the necessary mechanisms 

 exist. Thus we have the exudation of liquid water from the living and uninjured 

 plant. The phenomenon is very easily observed when young plants of Cabbage, 

 Maize or other Grasses, species of Alchemilla and many other plants have been 

 cultivated in flower-pots, the pots warmed up to 2o°-25°, and the leaves protected 



from too vigorous transpiration by being 

 covered with a large bell-glass'. After 

 a few hours, occasionally even after 15-zo 

 minutes, drops of water appear at certain 

 places at the apices and margins of the 

 leaves, especially on their teeth; these 

 gradually increase in size, and finally fall 

 off, whereupon new drops become formed. 

 Such drops of water occur particularly co- 

 piously at the apices of the leaves of many 

 Aroids, e. g. Colocasia antiquorum and 

 Calla Mthiopica. In the natural course 

 of events in the open air, the excretion of 

 drops on the leaves is easy to observe 

 when, after a hot day, the air becomes 

 cooler and moister as the sun goes down, 

 while the earth still retains its diurnal 

 warmth, and incites the roots to absorb 

 vigorously. It may then be directly seen 

 that drops of water are extruded at the 

 margins of the leaves of Potatoes, Grasses, 

 species of Alchemilla, and many other 

 plants. Much of the water which we find 

 early in the morning on the margins of 

 the leaves of many field and garden plants 

 in the form of large drops, and which are generally taken for drops of dew, is 

 really water excreted by the plants themselves. 



That as a matter of fact it is the root-pressure which drives the water forcibly 

 into the leaves, and expels it in the form of drops at appropriate places, may be 

 proved by fastening cut-off branches on the shorter limb of a wide glass tube, as in 

 Fig. 216, so that the cut surface dips into water, which is then forced into the shoot 

 by means of mercury poured into the other limb of the tube. If the leaves are 



Fig. 215. — The double walled vessel i, a stands on the tripod 

 d, d, which is furnished with the lamp /: water is contained 

 between the two zinc walls of the vesseL The flower-pot t, 

 with the plant p, stands on a support, i, i, and is covered by 

 the bcINglass g, g, which rests upon the hooks h, h. 



' All essential points respecting the extrusion of drops from leaves known up to that time were 

 collected in my ' pJlanzen-Physiologie'' (p. 23/), and the excretion of nectar was discussed at the 

 same time. 



