6? 



wood proper {xylem) ? for only these two alternatives are now 

 left us. If we take a plant which, when fully grown, has no 

 continuous pith in the centre of the stem but only a fibro-vas- 

 cular mass surrounded by cortex, e.g., a tree; and if we cut into 

 it all round the stem in the form of a complete ring, right down 

 to the interior of the layer whence annual additions are made to 

 the wood and bast, viz , the cambium, so as to remove all the 

 tissues for the width of one inch, and separate the cortex of the 

 upper part of the tree from that of the lower, but leave the 

 xylem bundles themselves untouched ; — now, if the water rose 

 by means of the bast, the tree should fade, because we have cut 

 off the supply which filled up the loss due to transpiration. 

 But the upper part of the tree, bearing the leaves, does not fade, 

 but continues to grow; the leaves continue fresh and green, and 

 fresh leaves are put forth ; the removal of the ring of cortical 

 tissue, then, does not interfere with the passage of liquid to the 

 leaves, and the only connection which can exist between them 

 and the roots is in the woody tissue of the plant, and hence 

 the water which passed from below upwards must have un- 

 doubtedly passed through the wood. It appears then, from 

 elaborate researches, that the vessels and fibres of the wood 

 take up the water absorbed by the roots and pass it on in the 

 thickness of their cell-walls, which consist, as we saw, of minute 

 solid particles separated by areas of water. 



But the question arises, how can liquid actually travel in the 

 substance of the lignified cell-walls ? To answer this we must 

 understand the precise nature and properties of a cell-wall when 

 lignified. These properties are not those of the ordinary un- 

 altered cellulose membrane ; for a cellulose wall has the power of 

 taking up a considerable amount of water between its micellae, 

 and of swelling up to a considerable degree. When it becomes 

 changed into one composed of lignin, however, it loses this power 

 of swelling up, and can take up a small amount of water between 

 its micellae, but only a relatively small amount, and it attains its 

 point of saturation more rapidly than in the case of the ordinary 

 cell-wall. It is not impermeable to water, on the contrary it takes 

 water up readily, and very readily parts with it again, but does not 



