690 MOLECULAR FORCES IN THE PLANT. 



It follows from these facts that, with the exception of times when the amount of 

 transpiration is small or when drops of water exude from the leaves, no root-pressure at 

 all exists when the plant is uninjured ; and that this pressure can only be detected when 

 transpiration and absorption have ceased or when they are very small. The exhaustion 

 of the root-stock of a strongly transpiring plant (as after it has been cut off) proves 

 rather that a plant with roots behaves in exactly the same way as a cut shoot. Just as 

 the latter absorbs water from a receiver, so the wood of the root-stock which has lost 

 water in consequence of transpiration above absorbs water from the cortical cells of the 

 root which obtain it by endosmose. From all this it still remains in doubt whether in 

 such cases the contents of the cortical cells of the root must not be left altogether out 

 of consideration, since it is possible that the conduction of water by the cell-walls alone, 

 reaches as far as the surface of the roots. 



(g) The parts of land-plants which are covered with a cuticle and in which transpira- 

 tion takes place appear to have no power of absorbing in any considerable quantity the 

 water by which they are moistened, such as the rain and dew which is deposited on the 

 leaves. As long as the tissues and leaves of uninjured plants with roots become turgid 

 and are supplied with water from below, any considerable absorption through the sur- 

 faces of the leaves themselves, even when they are thoroughly wetted^, is not to be 

 expected, since it is not easy to see where the water can go in cells that are already 

 gorged'-^. But even when a rooted plant has withered, it is still doubtful whether the 

 revival which takes place when its leaves are wetted depends on the absorption of water 

 by the leaves, since it is not impossible for an upward pressure to take place subse- 

 quently. Greatly withered shoots do not become turgid when placed in water, or do so 

 only very slowly unless the cut surface is immersed, and even in this case there is 

 doubt as to the absorption of water through the surfaces of the leaves. 



In harmony with this Duchartre found also^ that rooted plants {Hortensia, Helianthus 

 annuus)y which withered in the evening in consequence of the dryness of the earth 

 in the pot, did not recover or become turgid if copiously moistened by dew during a 

 whole night, the pots in which the roots spread being provided with a closed cover. 

 Epidendral Orchids, Tillandsias, &c. behave in the same way in this respect ; they 

 also absorb neither water nor aqueous vapour through their leaves, nor even in any 

 considerable quantity through the roots. The water which they require for their 

 transpiration and growth must be conveyed to them in the form of rain or dew which 

 moistens the root-envelope (velamen) or wounded surfaces*. 



When land-plants wither on a hot day and revive again in the evening, this is the 

 result of diminished transpiration with the decrease of heat and increase of the moisture 

 in the air in the evening, the activity of the roots continuing — not of any absorption of 

 aqueous vapour or dew through the leaves. Rain again revives withered plants not by 

 penetrating the leaves, but by moistening them and thus hindering further transpiration, 

 and conveying water to the roots, which they then conduct to the leaves. 



A simple experiment will afford much instruction to the student in these matters. 

 The pot in which a leafy plant is growing is enclosed in a glass or metal vessel provided 



^ On this subject see my Experimental-Physiologie, p. 159, 



^ Duchartre has neglected this obvious reflection in his researches (Bulletin de la Soc. Bot. de 

 France, Feb. 24, i860) ; in other respects also these experiments are very defective. 



3 Duchartre, I.e. 1857, pp. 940-946. 



* Duchartre, Experiences sur la vegetation des plantes epiphytes (Soc. Imp. et centrale d'horti- 

 culture, Jan, 1856, p. 67 ; and Comptes Rendus, 1868, vol. LXVII, p. 775). [This subject has been 

 recently investigated by Detmer (Theorie des Wurzeldrucks, 1877), by Boussingault (Les fonctions 

 physiques des feuilles, Agronomic, VI, 1878), and by Henslow (On the Absorption of Rain and Dew- 

 by the green parts of plants, Journ. Linn. Soc. XVII, 1879). It appears that leaves, like all other 

 parts of plants, will absorb water if they are immersed in it long enough, but there is no evidence 

 that the absorption of water, either as vapour or as liquid, is in any sense one of their functions,] 



