THE MOLECULAR FORCES IN PLANTS. 225 



is at once found that a disproportionately large quantity of aqueous 

 vapour has escaped during that time. Sudden shaking increases 

 considerably the transpiration of plants. I have often satisfied 

 myself of this. 3 



1 See Pfeffer, Handbuch der Pflanzenphysiologie, Bd. 1, pp. 135 and 141. 



2 On the relation between absorption and loss of water in plants, see espe- 

 cially Vescuie, Ann. d. Sc. Nat., 1876 and 1878. 



3 Literature : Unger, Anatomic, u. Physiolo.iie d. Pftanzen, 1855 ; Sachs, Hand- 

 bitch d. Experimentalphysiologie d. Pftanzen, 1865 ; Baranetzky, Botan. Zeitung, 

 1872; Wiesner, Sitzungsber. d. Akadem. d. Wins, in Wien, 1876, Octoberheft ; 

 Detmer, Beitrcifje zur Theorie d. Wurzeldmcks, Jena, 1877, p. 47 ; Kohl, Tran- 

 spiration d. Pflanzen, Braunschweig, 1886; Eberdt, Transpiration d. Pflanzen, 

 Marburg, 1C89. 



84. The Wood as a Tissue for the Conduction of Water, and the 

 Influence of Transpiration on the Movement of Water in 

 Plants. 



From the base of a very leafy branch of a tree or shrub (I ex- 

 perimented with Pavia rubra), without separating it from the 

 parent plant, we remove a ring of cortex about 5 cm. broad, and 

 extending inwards as far as the wood. 



The shoot remains fresh for a considerable time, notwithstand- 

 ing the fact that the leaves are actively transpiring, since con- 

 duction of water is not prevented by the ringing. Thus the bark 

 cannot be regarded as a tissue possessing any considerable im- 

 portance in connection with the conduction of water along the 

 stem; it is in the wood of the fibro vascular bundles that the water 

 travels. The dry or even partly destroyed pith in the middle of 

 woody stem-structures naturally plays no part in the movement of 

 water in the plant. 



Moreover, it is certain that not the whole of the wood conducts 

 water, but only the splint wood, the heart wood having become 

 incapable of conduction. Naturally only plants in which there is 

 a well-marked differentiation between splint and heart wood are 

 suitable for experiments on this subject. If, e.g., we select in 

 summer a Robinia trunk about 12-16 cm. in diameter, and saw 

 through it all round to the heart wood, the plant withers often on 

 the same day. If from a vigorous shoot of Rhus typhina we re- 

 move a ring of bark a few centimetres wide, the shoot rapidly 



P.P. Q 



