MOVEMENT OF WATER IN PLANTS. 6o2 



young leaves ainl intcrnodes is less efficacious in this respect ; if it is very thin as in many 

 ^^y>\.?L°-^y'";- leaves, especially those of water-plants, or altogether imperceptible 

 as m roots, these parts dry up very quickly in ordinary air; while an intermediate 

 condition is presented by the cuticularised outer layer of the epidermis of leaves and 

 young internodes. Ma contradistinction to this the evaporation is very small from hard 

 evergreen leaves, Cactus-stems &c., \Yhich are covered by a thick cuticular coating. 

 yinay be ass umed-that., in, j)lants„ pxQvided with a thick cuticle transpiration takes 

 £lace_ prmdpal]y. through the stomata, and is therefore dependent on their smaller or 

 Jiarger number ..and size. The evaporation does not in this case proceed from the 

 surface of the organ (or only to an imperceptible extent) but in its interior, 'viz. at the 

 places where the cells of the parenchyma bound the intercellular spaces. These spaces may 

 be supposed to be always at least nearly saturated with aqueous vapour ; but the vapour 

 will escape through the stomata with every increase of its tension or decrease of the 

 tension of the vapour without, and will thus give rise to the production of more vapour 

 in the inside. The_£rodiictiaiL of. vap.our in the intercellular spaces is moreover the more 

 abundant theJargELcthey are themselves, or in other words the larger the superficies of 

 cell- wall ..which bounds. 4,hem. This circumstance, and the much larger number of 

 stomata on the under side of the leaves, are clearly the reason why evaporation is gener- 

 ally so much more copious from it than from the upper side. Since water containing 

 any substance in solution evaporates more slowly than pure water, and the more slowly 

 the more concentrated and denser the solution, this force must also be considered among 

 the conditions which limit the transpiration of water from the sap of plants. It must 

 not however be forgotten that evaporation takes place only on the external surfaces 

 ^of the cell-walls of tissues, which on their part remove the water by imbibition from 

 _the._oeli-sap. 



The conditions now named which regulate transpiration are combined in the most 

 various ways, and not only cause different plants to show different amounts of transpir- 

 ation, but also the amount to be very different in the same plant at different times. 

 A definite statement cannot however be made of the total amount of transpiration, /. e. 

 of the quantity of water required by a plant during its period of vegetation, although 

 certain very variable limits can always be assigned to each species in this respect. 

 Two plants of the same species may, as any one may see, thrive equally well if one 

 grows in damp soil and dry air, the other in dry soil and damp air, the former thus 

 using up a large, the latter a small amount of water. In general the conditions of 

 transpiration which have been mentioned exhibit periodic variations related to the 

 meteorological distinction of day and night ; the temperature, the moisture of the air, 

 and light, are usually favourable to evaporation by day, unfavourable by night; but 

 under certain circumstances this condition may even be reversed. 



(f) Currents of IVater in the H^ood. Superficial cells or those which bound intercel- 

 lular spaces and lose water directly by evaporation, would very soon collapse and dry up 

 if they were not able again to replace that which they have lost. This can only take 

 place by the flow of water from the adjoining cellular tissue from which no evaporation 

 occurs ; but when this tissue is placed in the same condition as the former, it must also 

 compensate its loss from more distant layers of tissue, and these again from those which 

 are connected with the conducting organs or woody bundles which convey the water 

 from the roots. The question here presents itself whether this movement of water 

 within the succulent tissue (especially in the parenchyma of the leaves) is caused by 

 endosmose from cell to cell, or whether it does not occur at least principally along 

 the cell-walls, these latter forming the channels of communication between the woody 

 bundles and the surfaces where the evaporation takes place, the contents of the cells 

 being only incidentallv carried along with the transmitted fluid. 



The chief evidence of the fact that the currents of water m the roots, stem, and 

 branches caused by transpiration take place only in the wood, /'. e. in the lignified 

 xylem, has already been stated. It can be demonstrated in a more conspicuous manner 



