374 



LECTURE XVI. 



Potato plant mentioned above. All these relations come out clearly, moreover, if 

 the observations are graphically represented on a system of co-ordinates, in such 

 a manner that the days and hours of the observation are expressed on an abscissa 

 line, while the quantities discharged per hour appear as ordinates. The curve so 

 constructed facilitates a survey of the periodic fluctuations more than is the case with 

 a tabl ■. 



The liquid flowing out from the transverse section of the wood is by no means 

 pure water, but always contains small quantities of those salts which the roots 

 absorb from -the soil. With appropriate reagents, it is easy to detect potassium, 

 phosphoric acid, sulphuric acid, and lime in the discharged water; and small 



quantities of organic matters, such as sugar 

 and traces of proteids, may also be found 

 — substances which have evidently been 

 washed to a certain extent out of the wood 

 of the root-stock. 



The water flowing out at the transverse 

 section of the root-stock moves from below 

 upwards, and must therefore be set in mo-' 

 tion by pressure acting in opposition to 

 gravitation. This pressure, or, as I have 

 previously termed it, root-pressure, may, 

 however, be far more considerable than 

 is necessary for forcing the water out 

 of the short stump of a stem. Even 

 with smaller woody summer plants, as 

 Sunflower, Potato, Tobacco, Nettle, &c., 

 we may convince ourselves, with the aid 

 of the simple apparatus represented in 

 the accompanying figure, that the water 

 exudes from the transverse section of the 

 wood with a force capable of overcoming 

 the pressure of a column of mercury of 

 20-30 centimetres in height. The root- 

 pressure in old well-rooted stocks of the 

 Vine is much greater ; Hales found, for 

 instance, that the water is discharged from the transverse section of the wood with 

 a force which holds in equilibrium a column of mercury more than 100 centimetres 

 high, and some more recent observations show still greater energy on the part of the 

 root-pressure. 



If we now attempt to obtain an idea of the nature of this pressure, it is to be 

 noticed above all that the water at the transverse section of the wood wells forth 

 from the vessels and wood-cells, as may be directly observed. This water, how- 

 ever, is evidently taken up from the soil by the root-hairs, or by the outer cells of 

 the absorbing roots generally. Hence the problem to be answered may be stated 

 in this form : how, by means of absorption by the external cells of the root, can the 

 water obtain admittance to the cavities of the wood, and be pressed upwards in these 



FIG. 311. — Apparatus for observing the force with which 

 water is extruded from the root-stock at the cut surface of the 

 stem r. The glass tube R is first tightly connected with the 

 root-stock, and the manometer tube.*- then fixed firmly, by 

 means -of the cork k. R is then completely filled with water, 

 and closed by the upper cork k. Finally, mercury is poured 

 into r, so that g' stands higher than g from the first. The level 

 g' ascends above q according to the magnitude of the root- 

 pressure in each case. This apparatus is much more conve- 

 nient than those formerly employed. 



