LECTURE XIV. 



THE ASCENT OF WATER liST TRANSPIRING LAND-PLANTS. 



Plants require water for the growth of the younger organs, the cells of 

 which must be rendered turgid by endosmose. At the same time the water 

 taken up serves for the production of chemical compounds in nutrition ; and 

 if the surfaces of the shoot are in contact with the atmosphere, a part of the 

 absorbed water is exhaled in the form of vapour — a loss which must be again 

 made good by a fresh supply of water, if complete drying up is to be avoided. 

 It depends essentially upon the nature of a plant and its mode of life how the taking 

 up and giving off of the water are accomplished. It is in the first place obvious that 

 these matters will be otherwise in plants submerged in water, or in subterranean 

 plants, than in those which expose large evaporating surfaces in the air ; and in 

 these again it is important whether they are small and can endure an occasional 

 desiccation, as most Mosses and Lichens, or whether we have to do with larger 

 and more highly organised plants, the root-system of which extends in the moist 

 earth, and the assimilating foliage of which unfolds in the open air, and cannot 

 endure severe drying up. In the latter circumstances are found especially the erect 

 or climbing Ferns, Conifers, and Flowering plants. -On account of the great variety 

 of facts which here present themselves, it is advantageous to commence our con- 

 siderations on a typical case which allows us to recognise the important points 

 clearly on all sides. Let us picture. to ourselves for this purpose a tree, or even an 

 annual plant, the stem of which stands erect and unfolds its crown of foliage to 

 the air, i.e. plants of the common form, as met with in the Tobacco, Sun-flower, 

 ordinary Palms, Firs and Pines, Poplars, Oaks, or other forest trees. 



When in such plants the buds at the apex are to put forth shoots and unfold, 

 they require for the purpose a quantity of water which is nearly as large as the 

 organs themselves which are to be developed ; since the latter, in the fresh, fully 

 grown condition, consist of water to the extent of about nine-tenths or more of 

 their volume. This water, however, is absorbed far below, by the roots in the 

 earth, and must thus be conveyed to the growing shoots through the stem. As 

 soon as the leaves of the latter are unfolded, moreover, they constantly give off 

 aqueous vapour to the atmosphere; and, as may easily be observed on cut-off 

 shoots, they would completely dry up in a few days if water were not constantly 

 conveyed to them through the stem from the roots. Experience shows now that 

 in land-plants with large and thin leaves, the volume of water made use of for 



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