252 



LECTURE XV. 



from moist surfaces, and thus from the cell-walls which bound the intercellular 

 spaces of a leaf, is accelerated by a rise of temperature, transpiration must 

 increase with a rising temperature, always provided the stomata be open. According 

 to some observations made several years ago in my laboratory, it is not improbable 

 that the mere shaking of shoots facilitates the exit of water from the leaves ; this 

 is in so far worthy of closer investigation that the significance of the wind, and 

 the vigorous shaking of foliage-shoots effected by it, might be explained more 



in detail. That motion of the air 

 acts favourably on the transpi- 

 ration of leaves, as it does on eva- 

 poration from any moist body, is 

 obvious on a little reflection ; but the 

 shaking of shoots may, in addition, 

 favourably affect the mere expulsion 

 of the vapour from the intercellular 

 spaces. 



More than twenty years ago I fur- 

 ther confirmed the remarkable fact, 

 already in part noticed by Senebier, 

 that the transpiration from leaves 

 may also be altered by the presence 

 of materials dissolved in the water 

 which the roots take up*. Weak 

 solutions of those salts which plant? 

 employ as nutritive materials, — e.g. 

 potassium nitrate, gypsum, &c., — 

 poured on the soil in which the roots 

 are, bring about a noticeable retard- 

 ing of the transpiration. The same 

 takes place when uninjured roots, 

 developed in solutions of nutritive 

 materials, take up solutions of nutri- 

 tive salts instead of pure water. Cul- 

 tivated plants in a strongly manured 

 soil will therefore in general transpire 

 more feebly, and make use of less 

 water, than those in a soil poor in 

 food-matters. It is at present hardly 

 possible to give an entirely satis- 

 factory explanation of this fact ; since, although it is known that aqueous vapour 

 is given off less easily from a salt solution than from pure water, still the pheno- 



FIG. 204.— Apparatus for demonstrating the suction and transpiration 

 of a shoot under various external conditions. The shoot a is fastened 

 water-tight into the middle glass tube ; at d a thermometer is inserted 

 in the same way, which, by being pushed in, serves at the same time to 

 bring the level (c) of the water in the narrow tube at any time to its 

 original height again, when it has sunk by the suction of the shoot during 

 I — s minutes. If the shoot is large enough, and the apparatus not too 

 heavy, it may be placed on a scale and the loss in weight by transpiration 

 ^e determined at the same time. 



' On the changes in transpiration by the influence of salt solutions at the roots, cp. my treatise 

 in the paper, ' Die Landwirthschafil. Versuchsstationeity 1858, 1, p. 203, and 'Bot. Zeitung,' i860, 

 JJ'o. 14. More recent facts are quoted in Pfeffer's ' Pflanzen-physiologie,' Bd. 1, p. 151, 



