70 METABOLISM 



the alternation of thick and thin regions may be interpreted as a compromise 

 whereby the wall permits water to pass through whilst at the same time 

 maintaining its rigidity ; compromises such as this are frequently met with 

 in the architecture of the plant. From this point of view the narrow con- 

 necting bands of thickening deposit are quite comprehensible ; they render the 

 existence of broader thin regions possible and, at the same time, do not 

 materially impair the rigidity of the vessel (Schwendener, 1882 and 1892 ; 

 RoTHERT, 1899). We need not enter into a discussion here of the physiological 

 significance of the individual forms of thickening. 



Having now appreciated the relation existing between the structure of 

 the wall of the vessel and the function which the vessel itself fulfils, the next 

 question before us is the explanation of the chemical characteristics of the wall. 

 It is well known that the walls of aU vessels are lignified, that is to say, their 

 original cellulose reaction is greatly altered by the infiltration of aromatic sub- 

 stances (hadromal, Czapek, 1899). But our knowledge of the subject is very 

 limited ; we do not know how the physical characters of the cell-wall are 

 altered, nor whether the vessels are rendered fitter for the performance of their 

 functions by lignification. That lignification is by no means necessary is shown 

 by the fact that it takes place not only in vessels, but also in other elements 

 which have nothing to do with the transport of water. Nor in the present 

 condition of our knowledge are we able to form a judgement on the interpre- 

 tation to be given to the fact that the closing membranes of the bordered pits 

 are different in their chemical nature from the rest of the wall. 



Tracheae have received their name from the organs in animals known by 

 that title, because not only do they possess to a certain extent a similar struc- 

 ture, but also because they were supposed to have a similar function. For 

 long it was believed that they were the respiratory organs of the plant and 

 carried air in their interior. That this was an error was shown especially by the 

 researches of Von Hohnel (1879) and Bohm (1879), and since that time it has 

 been conclusively shown that the lumen of the vessel always contains water. 

 At certain times, e. g. during the growth of the vessels, and in trees during early 

 spring, when root-pressure is vigorous, the vessels are often completely filled 

 with water; but as soon as transpiration sets in, air is plentifully present in them. 

 Whence comes this air ? There are two possible means of entrance. It could 

 enter the vessel as air dissolved in the water already in the root, or it could in the 

 first instance diffuse through its wall in higher regions of the plant (Claussen, 

 1901). In both cases the airwould remain at first dissolved in the water which fills 

 the vessel. When, however, at the commencement of transpiration, the leaf-cells 

 demand more water from the vessel than it can supply, a vacuum must tend to be 

 formed in the vessel, and into that the air escapes from solution in the gaseous 

 state. Such air-bubbles will have a lower pressure than one atmosphere, and 

 will therefore act in a sucking manner ; water will be withdrawn from the 

 neighbouring tracheid, and in it, in turn, air-bubbles under low pressure will 

 appear. This negative pressure of the air in the vessels has been fully demon- 

 strated by V. Hohnel. He cut off branches under mercury from actively 

 transpiring trees and herbs, and observed how the metal was forced far up into 

 the lumina of the vessels by the external atmospheric pressure, overcoming 

 the very obvious capillary depression. The rarefaction of the air reaches 

 its maximum during the greatest transpiration ; but it may disappear again 

 entirely during the night owing to continuous entrance of water through the 

 root, whilst the vessels, when the air becomes dissolved once more, become full 

 of water. If the rarefaction of the air continues for a longer time, air from 

 outside enters through the walls of the vessels. When the air-pressure within and 

 without the vessel is by this means equalized, the vessels cannot be again com- 

 pletely filled with water, and further entrance of air, in the long run, will inter- 



