SECT. II 



PHYSIOLOGY 



203 



are 100 m., those of Sequoia gigantea 77-142 rn. in height. Osmotic 

 passage from cell to cell would bring about the movement of this 

 water far too slowly to cover the loss. The movement of water for 

 this purpose, or, as it is called, the transpiration stream, practi- 

 cally is confined to the woody portion of the vascular bundles, e.g. the 

 wood of trees. This is shown by a classical experiment (^^) repre- 

 sented in Fig. 189. At Z in the branch b all the tissues external 

 to the slender column of wood 

 have been removed. Since the j 



leaves of this branch remain as 

 fresh as those of the branch c, it 

 is evident that the transpiration 

 current must pass through the 



wood and not through the cortical 

 tissues. On the other hand, when 

 a short length of the wood is re- 

 moved from a stem without at the 

 same time unduly destroying the 

 continuity of the bark, the leaves 

 above the point of removal will 

 droop as quickly as in a twig cut 

 off from the stem. When a branch 

 is cut off and the cut surface is 

 placed in a solution of gelatine, 

 which penetrates for some distance 

 into the vessels and can then be 

 allowed to solidify, the wood will 

 be found to have lost its power 

 of conducting water. This shows 

 that the cavities of the vessels are 

 essential for water conduction. In 

 the living plant, however, the ves- 

 sels and tracheides always contain 

 air in addition to water, at least 

 when transpiration is active. 



In water plants and succulents, 

 in which little or no transpiration 



takes place, the xylem is correspondingly feebly developed. On the 

 other hand the transpiring leaf-blades have an extraordinarily rich 

 supply of vascular bundles ; these anastomose freely, so that any 

 particular point is sure to obtain sufficient water. The illustra- 

 tion (Fig. 190) gives some idea of this irrigation system of a leaf- 

 blade, but, since the finest bundles are only visible with the help of 

 the microscope and are not represented, the system is even more 

 complex. The conducting t-^acts in the stem leading to the leaves form, 

 especially in trees which grow in thickness, a wonderfully effective 



Fig. 189. — Hales' experiment to show the ascent 

 of the sap in the wood. Althougli the cortex 

 lias been entirely removed at ^, and the ■wood 

 alone left, the leaves of the branch b remain 

 as fresh as those on the uninjured branch c ; 

 X, vessel containing water. Facsimile of the 

 illustration in Hales' Vegetable Statics, 1727. 



