b6 



pressure exists between the air in the vessels and the external air. 

 In Avhat way is this negative pressure brought about ? First let 

 us take as an example for our consideration a low-growing plant, 

 such as Alchemilla, which in the night transpires very little 

 water and absorbs a good deal — so much so, that the water is 

 forced by filtration under pressure into the vessels, and becomes 

 exuded as drops of water along the margin of the leaves in the 

 morning by means of special arrangements known as water- 

 pores, of which more hereafter. During the daytime transpir- 

 ation becomes active, and as it proceeds the water is withdrawn 

 from the cavities of the vessels to meet the demand, and conse- 

 quently these cavities become empty. Transpiration has the 

 effect, then, of tending to produce a vacuum, for a small amount 

 of air present in the cavities along with the liquid becomes much 

 expanded or rarefied, and exists at a lower, more diminished pres- 

 sure than that of the external air, i. e., at a. negative pressure. 

 When night comes again, the activity of the root absorption 

 forces water into the vessel cavities and compresses the rarefied air 

 so as to bring it back to a pressure equal to that of the air outside. 

 What happens to this plant daily, takes place also in large plants 

 such as trees, yearly. In the spring of the year, before the de- 

 velopment of the leaves has taken place, the cavities of the 

 vessels contain a considerable quantity of water, and also some 

 air at normal pressure. When transpiration sets in and becomes 

 active, this water is gradually withdrawn, and the remaining 

 air expands, owing to the creation of a partial vacuum, and so 

 it comes to have a lower pressure than that of the external air, 

 and this is the condition in which we found it existing upon 

 section in the summer. The presence of this negative pressure 

 demonstrates the fact that the cavities of the wood-cells and 

 vessels are completely shut off from the external air, and have 

 no direct communication with it through the openings of the 

 stomata, as was formerly believed ; for if such an opening to the 

 exterior did exist, this negative pressure could not be present. 



When we submit sections of the parts of the plant to micro- 

 scopic examination, we can see how this complete closure is 

 brought about. We shall then find that the vessels of the wood 



