THE MOVEMENT OF WATER IN PLANTS. 97 



when both cortical parenchyma and pith were removed. It is 

 evident that, in spite of the removal of these tissues, the leaves 

 still continue to receive from the roots a supply of water which 

 is sufficient to compensate their loss by transpiration, and thus 

 to enable them to remain fresh and turgid. This supply of 

 water must necessarily ascend to them through the wood of 

 the stem, since this is the only tissue which still connects them 

 with the roots. 



It is, then, the wood by which the water which is required 

 to make up for the loss by transpiration, is conveyed from the 

 roots to the leaves. In annual plants, the whole of the wood 

 conducts water, but in perennial plants it is only the younger 

 wood. This is proved by an experiment made by Knight 

 and Dutrochet. They cut a ring of tissue out of the stem 

 of an Oak to such a depth that the younger wood (alburnum) 

 was removed and the older wood (duramen) was laid bare. 

 The leaves soon began to wither, and the tree subsequently 

 died. When the wood once becomes dry it loses its capacity 

 for imbibition, and can therefore no longer serve for the con- 

 duction of water in the plant. 



Since we know that the water travels upwards through 

 the young wood, and since it is stated that at the time 

 when the current is most active, the cavities of the cells and 

 vessels contain no water, we must conclude that it travels in 

 the substance of their cell-walls. This conclusion is quite in 

 harmony with what we know of the properties of lignified 

 cell-walls. We learnt, in the second lecture, that such cell- 

 walls readily take up water, without, however, exhibiting any 

 perceptible swelling, so that their maximum saturation or 

 imbibition is very soon attained. They part with water as 

 readily as they take it up, so that a very small force will set 

 up a current. This is well shewn by the following experiment 

 of Sachs. If a portion, two or three feet long, of the stem of 

 a young Fir be taken in the winter, when it is saturated with 

 water, and the two ends be cut smooth with a knife, they will 

 be found to be quite dry when the piece of stem is held verti- 

 cally. If now the upper cut surface be wetted by means of 

 a brush, the lower cut surface will be seen to become moist 



v. 7 



