Q.'/O LECTURE XVI. 



as promoting the movement of the imbibed water in the lignified cell-walls may 

 well be conjectured, but cannot be proved. 



If we bear in mind the phenomena observed on warming and coolingj a piece of 

 wood cut off in winter, it is clear that within the stem of a living tree, movements of 

 water will also occur in the interior of the wood, through the unequal warming at 

 different heights which must necessarily occur with sudden changes of temperature. 

 When, for example, the sun shines on a tree after a cold night, the thinner branches 

 are more rapidly warmed than the thick stem, and the water is driven from them 

 into the latter; and conversely, with the increasing cold of night the branches become 

 more rapidly cooled than the thick stem, and take up water from the latter. With 

 a rise of temperature in winter, tension in the wood generally must occur, since the 

 air-bubbles in the wood-cells on becoming warmed tend to force the water through 

 the walls, which is prevented by the cortex. If, however, a hole penetrating as far as 

 the alburnum is made with an auger, and an exit-tube, bent downwards and opening 

 at the bottom of a flask, is placed in it, these effects of changes of temperature may 

 be made visible. As the temperature rises, water flows out of the hole into the 

 flask; this has simply been expelled by the expanding air-bubbles of the wood. 

 On the recurrence of cold weather, however, the air-bubbles contract; and the 

 consequence of this is that the water which had flowed out into the flask is 

 again sucked up through the tube into the wood. Although, as was shown above, 

 the water expelled by the rise of temperature is always only a small portion of that 

 present in the wood, nevertheless several litres of water may be driven out from 

 a large tree in an experiment of this kind, if the increase of temperature is suf- 

 ficient, since the water contained in the wood of a large tree may certainly amount 

 to 'hundreds of litres. Moreover, the phenomenon need not be confined to the 

 winter; it may continue also, in the spring, and even in summer (e.g. with the 

 Birch), since it only depends upon how much water is contained in the wood, and 

 upon the corresponding changes of temperature. The characteristic fact of this 

 kind of bleeding, however, simply lies in that it takes place even in winter, and 

 independent of the period of vegetation. 



The second kind of bleeding, or the weeping of the root-stock, is distinguished 

 from that hitherto considered, in that it only takes place during the period of proper 

 vegetative activity, when the roots have already commenced to absorb water from 

 the warm soil. This phenomenon has been observed in the Vine from of old. 

 In spring, when the soil is to a certain extent warmed and the roots incited to 

 activity, if a Vine is cut through close to the earth, or even higher, a quantity of 

 clear water instantly exudes from the wood ; and this outflow of water may continue 

 for many days, and, as I know by personal experience, may yield several litres 

 of water. The same happens with many trees in the spring time. Birch-stems, for 

 example, sawn across a little above the soil, send forth streams of water, coming from 

 the alburnum, which exude from the root-stock for weeks. This bleeding is, 

 moreover, by no means confined to woody plants in the narrower sense of the word, 

 as is proved by the old custom of the Mexicans of obtaining enormous quantities of 

 their national drink, pulque, from the huge plants of Agave americana (the so-called 

 'hundred-years Aloe'), by cutting out the so-called 'heart' or leaf-bud, within 

 the rosette of leaves, upon which water, driven up from the roots, is excreted 



