MOVEMENT OF WATER IN PLANTS. 613 



a tobacco-plant amounted in five days to 200 cubic cm., while the root-stock exuded 

 only 15-7 cubic cm. In the same manner in Cucurbita Pepo (when much withered) the 

 amount absorbed was 14 cubic cm., the exudation from the root-stock only 11-4 cubic 

 cm. The withered upper part of a sunflower absorbed in a few days 95 cubic cm., 

 while the root-stock exuded only 52*9 cubic cm. The result is also the same when 

 the relative amounts which extend over a shorter time are compared. 



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 is exerted only after 

 evaporation 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 evaporation 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 suction of the cell-walls merely, due to 

 imbibition or surface-action, reaches as far as the surface of the roots. 



{g) The parts of land-plants which are covered with a cuticle and which serve for 

 transpiration 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, if they are already quite moist, 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 plant has withered, it is still doubtful whether its revival depends on the ab- 

 sorption of water by the leaves, since it is not impossible for an upward pressure to take 

 place subsequently. Greatly withered shoots do not under such circumstances become 

 turgid or do so only very slowly unless the cut surface is placed in water, 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 rooting plants (Hortensia, Helian- 

 thus amiuus), which wither 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 con- 

 siderable quantity through the roots. The water which they require for their trans- 

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

 moistens the root-envelopes 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 



' 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. 



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



■' Duchartre, Experiences sur la vegetation des plantes epiphytes (Soc. Imp. et ccntrale d horH^ 

 cultme, Jan. 1856, p. 67; and Comptes Rendus, 1868, vol. LXVII, p. 775). 



