50 



METABOLISM 



cut ends of latex tubes, sieve tubes, and similar elongated elements, which, 

 in consequence, lose their turgescence. This particular instance, however, is not 

 apposite in this relation. Similarly the contents of the vessels may be squeezed 

 out by the pressure of surrounding parenchyma, more especially if the vessels be 

 young ; at a later stage the secondary thickenings in their walls renders this im- 

 possible. Nevertheless it is obvious that only a small amount of sap can pass out 

 in this way. The following are the amounts of water which Hofmeister (1862) 

 found were ejected from the root-systems of Urtica urens and Solanum nigrum : — 



These observations are specially valuable in consequence of the fact that 



the volume of the roots from which the water was 

 excreted was noted as well. The table shows that 

 in a few days the root gives off several times its own 

 volume of water. During that time it must have 

 been absorbing new supplies of water from the soil, 

 and it can, after even a longer period of activity, 

 still maintain its original amount of liquid contents. 

 Similar excretions occur elsewhere, not only 

 directly from roots, but also from stems and 

 branches, if these be cut off from the plant or 

 if borings be made down to the woody cylin- 

 der. The phenomenon of ' weeping ' or ' bleed- 

 ing', as the gardener terms it, which occurs in 

 spring-time, notably in the vine, when the stem is 

 cut, is well known to every one. This phenomenon 

 we may also designate by the term ' bleeding '. It 

 has long been known that the sap is ejected from 

 the plant often with very considerable force (' bleed- 

 ing-pressure ' or ' root-pressure '). Long ago, 

 physiologists, e. g. Hales (1748), measured this pres- 

 sure in essentially the same way as we do to-day. 

 A glass tube, bent twice in the form of a U, is 

 fitted to the stump of a root (Fig. 12) and filled 

 with water just above the cut surface and then 

 closed with mercury. By noting the height to 

 which the mercury is driven in the open leg of the 

 tube it is possible to estimate the amount of root- 

 pressure. 



Our next task must be to attempt a more exact 

 estimate of the quantity, quality, and pressure of the sap expressed from such 

 wounds. 



On submitting the sap which has been collected to analysis, we find it to 

 consist of water in which are always dissolved organic and inorganic substances 

 although in very varying proportions. The extremely watery sap of the potato, 

 the sunflower, and the vine, contains from i per cent, to 3 per cent, of solid in 

 solution, of which two-thirds in the case of the vine, half in the sunflower, and 

 one-third in the case of the potato is organic in its nature. The inorganic salts 

 of the sap are exactly the same as those which we have already met with in the 

 plant, while among the organic compounds, acids, proteids, and especially 

 sugars occur. In concentrated sap the various sugars markedly predominate ; 



Fig. 12. The glass tube, jp-, is fixed 

 by means of rubber tubing, c, to the 

 stump of a Dahlia^ s, and filled with 

 water ( W) and mercury {Q). (^From 

 the Bonn Textbook.) 



