(i^d 



MOLECULAR FORCES IN THE PLANT. 



certain time of nearly the same size, and the stem and the channels of the 

 current (woody bundles) which traverse it consequently retain their diameter 

 unchanged. 



The movements of water caused by growth as well as those induced by 

 evaporation have this in common, that their direction is towards the places where 

 they are required. If growth or transpiration begins at a certain time at a 

 definite spot, the nearest portions of the tissue give up their water first of all, 

 then the more distant ones, the organs at the greatest distance, generally the 

 roots, absorbing water from without. The movement therefore propagates itself 



continually further and further from the point to which 

 it tends, and finally over the whole plant to the root. 

 The kind of motion may therefore — without consider- 

 ing for the moment its actual causes — be described 

 as a process of suction. This is especially evident 

 in leafy stems and branches which, having been cut 

 off and placed with their cut surface in water, suck 

 up as much water through their woody bundles as 

 is required for transpiration and for the unfolding of 

 fresh leaves, unassisted in this case by any pressure 

 from below. 



Another kind of motion of water in the plant, 

 depending not on suction but on pressure from below, 

 is caused by the roots, and is altogether independent 

 of the use of the water for the purpose of growth or 

 of transpiration. If the woody stem of a land-plant 

 is cut through above the root, the root being attached 

 to the ground in the ordinary manner, and if the 

 ground is damp and sufficiently warm, water exudes 

 from the transverse section of the stem either at once 

 or after some time, the current continuing for days, 

 and the quantity of water which flows out amounting 

 sometimes to many times the volume of the root. 

 This current of water, which rises through the wood 

 and especially in the vessels, can only be induced by 

 a pressure existing in the lower parts of the root. If 

 a manometer of a proper form is fixed in the section 

 (Fig. 467), it shows that even in smaller plants with but little wood (as Tobacco, 

 Maize, the Stinging Netde, &c.) the water which exudes stands at a pressure 

 which holds in equilibrium a column of mercury several centimetres in height ; 

 while in some woody plants, as for instance the Vine, this pressure may amount 

 to 76 cm. (or one atmospheric pressure). 



In many plants of small height this root-pressure is observable from the fact 

 that water exudes at particular points of the leaves in the form of drops, pro- 

 vided that the internal supply of w^ater is nowhere diminished by powerful transpira- 

 tion, and the pressure thus_ removed. Thus drops of water appear abundantly 

 and repeatedly on the margins and apices of the leaves of many Grasses (especially 



Fig. 467.— Apparatus for observing the 

 force with which water escapes under root- 

 pressure from the transverse section of a 

 stem r. The glass tube R is first of all 

 firmly fastened to the stem, and the tube r 

 then fixed into it by the cork A. H is com- 

 pletely filled with water, the upper cork i 

 then fixed in it, and mercury poured into 

 the tube r so as to stand from the first 

 higher at g' than at g, the level ^' rising 

 above ^ according to the intensity of the 

 root-pressure. Tiie apparatus is much 

 more convenient to handle than that hither- 

 to in use. 



