PHYSIOLOGY 203 



dentally, remained nevertheless perfectly fresh. The adjoining 

 figure, taken from one of the first books in which the vital processes 

 in plants were described (ESSAYS ON VEGETABLE STATICS, by STEPHEN 

 HALES, 1727), shows the method employed in proving this fact experi- 

 mentally (Fig. 188). At Z in the branch b all the tissues external to 

 the slender column of. wood have been removed. Since the 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 removed 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 on a twig cut off from the stem. 

 It has also been shown by experiment that in herbaceous plants the 

 vascular portions of the bundles provide for the conduction of the 

 ascending currents. 



As can be demonstrated by spectroscopical analysis, a dilute solution of li'tliiuni 

 nitrate taken up by an uninjured plant first ascends in the wood before it passes 

 laterally into the other tissues. By means of the same solution, MAcNAB, PFITZEI:, 

 and SACHS determined the velocity of the movement of the transpiration current, 

 which naturally varies according to the plant and the effect of external conditions 

 upon transpiration ; under favourable circumstances it attains a rate of 1-2 metres 

 an hour. The method of showing the exclusive share of the wood in the con- 

 duction of the water, and, also, of determining the maximum velocity of the 

 transpiration current, from observations based on the path and rate of movement 

 of a coloured solution taken up by a plant, is not free from objection, for the 

 colouring 'matter does not pass through the stem at the same rate as the water 

 in which it is dissolved, but is drawn out and held back by the cells. The 

 employment of coloured solutions will, however, be found instructive for merely 

 >lio\\ing the course of the transpiration current. The transparent stems of the 

 Balsam, Imputiens parviflora, and the white- floral leaves of Lilies, Camellias, 

 ilock Orange, etc., in which the coloured vascular system will stand out as a fine 

 iirnvork, are especially adapted for such an experiment. 



In water plants and succulents, in which little or no transpiration 

 takes place, the xylem is correspondingly feebly developed. In land 

 plants, on the other hand, and especially in trees with abundant 

 foliage, the wood attains a much greater development. All the wood, 

 however, of a large stem does not take part in the task of water- 

 conduction, but only the younger, outer rings. Where there is a 

 distinction between heart- and sap-wood, the heart -wood takes no 

 part in the conduction of the water. 



The character of the forces which cause the ascent of the tran- 

 spiration current is still unexplained. Transpiration itself makes a 

 place for the inflowing water. By the removal of the imbibition 

 water from the cell wall, which is replaced at the expense of the 

 supply contained in the osmotically active cell, force is exerted which 

 causes the water in the adjacent elements of the xylem to move 



