6oo 



MOLECULAR FORCES IN THE PLANT. 



therefore propagates itself continually further and further from the point to which it 

 tends, and finally over the whole plant to the medium which surrounds the root. 

 The kind of motion may therefore — without considering for the moment its actual 

 causes — be described as a process of suction. Tliis is especially evident in leafy 

 stems and branches which, having been cut off and placed with their cut surface in 

 waier, suck up as much water through their woody bundles as is required for transpir- 

 ation 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 evaporation. 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 suffici- 

 ently warm, water exudes from the trans- 

 verse 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 cur- 

 rent of water, which rises in wood as 

 well as in glass tubes, can only be in- 

 duced by a pressure exerted on the lower 

 parts of the root. If a manometer of a 

 proper form is fixed in the section (Fig. 

 438), it, shows that even in smaller plants 

 with but litde wood (as tobacco, maize, the stinging nettle, &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 water is nowhere diminished by powerful evapor- 

 ation, 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 

 striking in the maize), Aroideae, Alchemilla^ &c., when transpiration is diminished 



KiG. 438. — Apparatus for observing the force with 

 whicli water escapes under root-pressure from tlie trans- 

 verse section of a stem r. The glass tube H is first of all 

 firmly fastened to the stem, and the tube r then fixed 

 into it by the cork Jt. Ji is completely filled with water, 

 the upper cork A then fixed in it, and mercury poured 

 into the tube r so as to stand from the first higher at g' 

 than at </, the level g' rising above g according to the in- 

 tensity of the root-pressure. The apparatus is much 

 more convenient to handle than that hitherto in use. 



* According to Duchartre, De la Rue, and Rosanoff, the exudation usually takes place through 

 the stomata, which are either developed in a peculiar manner, or are very large, or, possessing 

 the ordinary form, are accumulated at these places. De Bary remarks in connection with this : — 

 'If water is forced into the wood of a branch of a plant adapted to the purpose, e.g. Fuchsia 



