362 THE VASCULAR TISSUES AND TRANSPIRATION. 



CONNECTION BETWEEN THE STRUCTURE OF THE VASCULAR TISSUES 



AND TRANSPIRATION 



It is naturally to be expected that between the contrivances regulating transpira- 

 tion in the immediate neighbourhood of the green tissue, and those mechanisms which 

 effect the transport of the crude sap from the roots, through the stem and branches, 

 up to the region of this transpiring tissue, a mutual co-operation will exist. 



Where much water is exhaled from the surface, much water must be supplied, 

 and in tracts leading to extensive and strongly-transpiring leaf-blades, the fluid 

 moves more quickly than in a conducting apparatus leading to green tissue, which 

 transpires but slowly and to a small extent. In pines, whose stiff acicular leaves 

 transpire but little, the raw food-sap moves much more sluggishly than is the case 

 with maples, whose flat leaves give off large quantities of water in the form of 

 vapour. The quickest movement, however, is to be found in twining and climbing 

 plants, whose stems, a few centimetres in thickness, may attain to a length 

 exceeding 100 metres. This is the case in those peculiar climbing palms, which at 

 first wind over the ground in numerous snake-like coils, and then rise to the tops 

 of the highest trees, and unfold their leaves there in the sunshine. Climbing palms 

 (Rotang) are known whose stems actually attain a length of ISO metres, and which, 

 when they have reached the summit of the trees after numerous windings, become 

 erect and extend their larger pinnate leaves just like the straight-stemmed palms. 

 The illustration opposite (fig. 94) depicts in the background the edge of a wood up 

 whose trees have climbed examples of such a species of Rotang. 



Many hours of the day may pass, when, on account of a clouded sky and the 

 great humidity of the air, the transpiration in the wide-spreading leaves above the 

 tops of the trees will be extremely little; but when the sun shines brightly and the 

 leaves become thoroughly warmed, a large quantity of water vapour must be 

 exhaled in a very short time. This quantity of water must be replaced, and very 

 quickly, but by means of a stem 180 metres long and only some centimetres thick. 

 In order to render the replacement possible, everything which might hinder the 

 rapid movement of the water and its dissolved food-stuffs on its long journey, 

 especially the resistance of the conducting tubes, must be minimized as much as 

 possible. The forward movement of fluids in a channel is, however, rendered 

 more difficult as the tube narrows, because in a narrower tube a relatively larger 

 amount of the fluid adheres to the inner surface, and therefore it is necessary, in 

 order to obtain a rapid movement, that this adhesion be reduced as far as possible. 

 This is most simply effected by widening the channel, since the adherent surface 

 is thus diminished in comparison with the large amount of the fluid passing 

 through. As a matter of fact, in the stems of climbing palms relatively verj' wide 

 tubes are to be seen, through which a large quantity of fluid can be brought 

 from the roots to the transpiring leaf-surfaces in a very short time, and this 

 actually occurs. The climbing palm, Calamus angustifolius, has conducting tubes 



