1S6 



BOTANY 



BUNDLES. The woody elements which thus conduct the water have 

 no protoplasm ; they are to be regarded as dead cells, in which the 



last office of the protoplasm was 

 h to give the walls their peculiar 



structure. 



III. The Transpiration Cur- 

 rent. — It has long been known 

 that the ascending transpiration 

 current in woody plants, which is 

 directed to the points of greatest 

 consumption, flows solely through 

 the wood. It had been observed 

 that plants from which portions 

 of the cortex had been removed, 

 either purposely or accidentally, 

 remained nevertheless perfectly 

 fresh. The adjoining figure, taken 

 from one of the first books in 

 which the vital processes in plants 

 were accurately described (Essays 

 on Vegetable Statics, by 

 Stephen Hales, 1727), shows the 

 method employed in proving this 

 fact experimentally (Fig. 175). 

 At Z in the branch b all the 

 tissues external to the slender 

 wood have been removed. Since 

 the leaves of this branch remain 

 as fresh as those of the branch c, 

 it is evident that the transpira- 

 tion 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. 



Fig. 175. — Hales' experiment to show the ascent 

 of the sap in the wood. Although the cortex 

 has been entirely removed at Z, and the wood 

 alone left, the leaves of the branch & remain 

 as fresh as those on the uninjured branch c ; 

 x, vessel containing water. Facsimile of the 

 illustration in Hales' Vegetable Statics, 

 1727. 



As Sachs demonstrated by spectroscopical analysis, a dilute solution of lithium 

 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, Pfitzee 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. This method of showing the exclusive share of the wood in the con- 



