THE CONDUCTION OF WATER. I 



47 



of water from cell to cell in the higher plant cannot possibly suffice to replace 

 the loss due to transpiration, and experiments have confirmed this conclusion. 

 WESTERMAIER (1884) arranged strips of somewhat flaccid parenchyma taken 

 from the water-bearing tissue of Peperomia and Tradescantia in such a way 

 that one side only was in contact with water, and observed to what distance 

 from the surface of the water the cells regained their turgescence. Although 

 the nature of the external conditions made transpiration almost impossible, 

 the height to which the water ascended never amounted to more than a few 

 centimetres. Even the cells which were more than from about two to four 

 centimetres distant from the surface of the water were unable, by means of 

 osmotic suction, to attain their normal water capacity. From experiments of 

 a similar nature, REINKE (1902) concluded that parenchymatous tissues of 

 submerged plants, if one side be immersed in water, become dried up about two 

 millimetres above the water level. Such experiments as these, coupled with 

 observations on small multicellular plants, force us to the conclusion that in 

 larger plants, trees especially, where the height of ascent of water is to be 

 measured not by centimetres but by metres, the movement of water cannot 

 possibly be effected by its transference from one living cell to another, but that 

 conducting organs with special structure and activity must be forthcoming by 

 means of which transport of water in quantity may be carried out. 



These organs are the vessels. That this is the case may in the first place be 

 concluded on purely anatomical grounds. The contents of the vessels consist, at 

 least in part, of water ; their elongated form and the continuity of their lumina 

 for long distances point emphatically to the conduction of water as their func- 

 tion. Not less suggestive is their distribution. They begin to appear in the 

 centre of each root about the same distance from the apex as the roothairs 

 do on the exterior ; and these roothairs are, as we have already seen, the 

 water-absorbing organs. They stretch from this point upwards, uniting in 

 their progress with side conduits from every lateral root, increasing in number 

 and in total transverse section as they progress from apex to base. On reaching 

 the stem they pass outwards into every branch, every twig, and every petiole. 

 In all these organs the vessels are collected together into a few strands, but in 

 the foliage leaves, the organs of transpiration, we find these cords resolving 

 themselves into stouter or weaker subdivisions which are distributed through- 

 out the leaf-blade in the form of a complicated network, so that every trans- 

 piring cell is in contact with a vessel either directly, or indirectly through the 

 medium of a few parenchymatous cells. We thus arrive at the important con- 

 clusion, already recognized as fundamental on other grounds, viz. that trans- 

 port of water from cell to cell is extremely limited, if indeed the cell bordering 

 on a vessel can abstract water from it. 



Physiological evidence as to the function of vessels may be most readily 

 found in trees, since, owing to their often great height heavy demands are 

 made on the vessels in such plants. Further, it is not unusually the case that 

 both on the main trunk and on the branches there are regions of considerable 

 extent which possess no lateral organs of transpiration and which are pre- 

 vented from transpiring by their corky coverings. In such situations transport 

 of water only, and no evaporation, can take place ; moreover, by breaking the 

 continuity of the tissues in succession, we can determine with certainty which 

 anatomical system is especially concerned in water conduction. Comparative 

 examination shows us that the pith has nothing to do with water transport, 

 seeing that in many cases it is absent altogether or consists of dried-up cells 

 filled with air, or of parenchyma, with whose feeble power to act as a trans- 

 porter of water we are already acquainted. It is quite otherwise with the 

 cortex and phloem ; there is no want of elongated elements here running con- 

 tinuously for long distances, e. g. collenchymatous and sclerenchymatous tissue 



