65 



to the roots, intercepted, but not interrupted, by the solid micellae : 

 and the whole question of its passage in these is a simple question 

 of hydrostatic equilibrium; — the smallest disturbance, causing with- 

 drawal of water at one point, is sufficient to cause a rush of water 

 at another to supply its place. 



That the lignified cells of the wood serve for the conduction of 

 water, is also confirmed by the fact that submerged water plants, 

 which can have no transpiration, and lose no water at their surface, 

 have no lignified elements in their wood. In these plants we find 

 that the fibro-vascular system is very feebly developed, and almost 

 absent, although in land plants closely allied to them we find a 

 well-defined fibro-vascular system. This mode of conduction of 

 liquids is confined then to plants having a well-developed fibro- 

 vascular system ; and as the physiological significance, and one of 

 the uses, of a great fibro-vascular system, is to afford a supply of 

 the water absorbed by the roots to the leaves, we find it most 

 developed in plants having a well-developed leaf system. 



So then, at the leaves, we find a considerable demand for wa- 

 tery vapour which must be supplied from below ; this watery 

 vapour passes between the micellae in the thickness of the woody 

 walls of the vessels, and makes its way to the cell-walls of the 

 leaves. The fibro-vascular system of the plant when the plant 

 . is entire is a completely closed one, as are the blood vessels 

 of animals ; under ordinary circumstances we find no communi- 

 cation between the external air and the interior of the fibro- 

 vascular system. For if we make a section of the stem of an 

 actively transpiring plant under mercury, we find that upon 

 section the mercury is absorbed into it (or better still, use 

 solution of a salt of lithium, taking care to make the section 

 under the surface of the fluid) ; hence, the air in the cavities of 

 the vessels is at a lower pressure than that of the atmosphere, 

 that is to say, it exists at a minus or negative pressure. This 

 experiment, then, demonstrates the truth of the statement that 

 the vessels must be completely closed, otherwise there would 

 have been an equality of pressure between air in the vessels 

 and the external atmosphere. 



A negative pressure, then, indicates that a marked difference of 



