Il8 LECTURE VII. 



case. In a previous lecture we learned that in certain plants, 

 such as Alchemilla vulgaris for example, drops of water are to 

 be found in the early morning on the leaves, and that these 

 drops have been exuded by the action of the root-pressure. 

 During the night the roots have been absorbing water actively, 

 so actively in fact that the tissues have become gorged, the 

 vessels more or less filled, and drops of water have been 

 forced out at the margins of the leaves. When the sun shines 

 upon the plant during the day, it transpires actively and the 

 leaves soon begin to draw upon the water which is contained 

 in the cavities of the vessels. As the water is withdrawn from 

 these, a partial vacuum is produced : the gases which are 

 already present in them, and those which pass into them by 

 diffusion, expand, and are at a very much lower pressure than 

 that of the atmosphere. If now the stem of the plant is cut 

 through under mercury, the mercury must necessarily rise into 

 the cavities of the vessels until the gases which they contain are 

 at a pressure approximately equal to that of the atmosphere. 

 Exactly this process goes on also in trees, but on an extended 

 scale. In the spring the vessels of trees are just in the con- 

 dition in which those of Alchemilla are in the early morning: 

 they contain water and bubbles of gases. When transpiration 

 commences after the leaves have unfolded, the water is 

 gradually withdrawn from the vessels, and, as the summer 

 advances, the gases in the vessels become more and more 

 rarefied, as do those in the vessels of Alchemilla during the 

 day. It is not until the fall of the leaves that the cavities of 

 the vessels begin to contain water, and that the gases are 

 reduced to their normal volume. 



The effect of the existence of this diminished, or negative, 

 pressure in the vessels finds its expression in various ways. 

 In the first place, it must facilitate the filtration of water from 

 the innermost parenchymatous cells of the root into the wood. 

 That this is the case is shewn by the experiments of Vesque, 

 to which reference was made in a previous lecture (p. 52). He 

 found that a sudden rise in the temperature of the air checked 

 the absorption of water by the roots. The interpretation of this 

 fact is that the sudden rise of temperature causes an expan- 



