678 MOLECULAR FORCES IN THE PLANT. 



much appears certain, that the ultimate force concerned is diffusion (in the broadest 

 sense of the term). But since in the living plant this force acts under conditions widely 

 different from those in operation in artificial apparatus, we are compelled on all essential 

 points to draw our conclusions as to the internal processes from the careful study of 

 the phenomena in the plants themselves. Our space will however only permit us to 

 refer to these in general terms. The main result of the investigations hitherto made 

 is to maintain the distinction between the different causes of motion in the fluids of the 

 plant to which we have already alluded, until a more thorough knowledge justifies some 

 other interpretation. What follows is less for the purpose of explaining the phenomena 

 than of illustrating by examples what has already been said. 



(a) The slow movement of water caused merely by Growth and Assimilation is 

 seen in its simplest form in unicellular Fungi and Algae and in those in which the cells 

 are arranged in rows and plates, and in germinating spores and pollen-grains ; since in 

 these cases the growing and assimilating cells absorb the water which they require im- 

 mediately from their moist environment. That this is caused by the imbibing power 

 of the cell-wall and of the protoplasm as well as by endosmose {i. e. the attraction of 

 the dissolved substances within the cell for water), is certain, although we have not 

 yet sufficient knowledge of the exact mode in which these processes go on. On the 

 other hand, in plants which consist of masses of tissue the young growing parts 

 withdraw the water of vegetation from the older mature parts, and these latter become 

 in consequence empty if they receive no fresh supply from without. This is seen 

 clearly when tubers, bulbs, trunks of trees which have been cut down, &c.^ put out 

 buds in ordinary moderately dry air, and thus gradually lose the water they have 

 contained ^. 



{b) Transpiration ^ — /'. e. the evaporation of water from cells and masses of tissue — is 

 produced and modified by external and internal conditions and causes. Among ex- 

 ternal causes those must first be noted which produce evaporation from moist surfaces, 

 such as the relative temperature and dryness of the air and that of the transpiring 

 tissue itself. Evaporation will generally increase as the temperature of the surrounding 

 air rises and its degree of saturation consequently decreases ; and this must for our 

 purpose be considered the most direct measure of the greater or less tendency to 

 evaporation. It must not however be expected that the amount of evaporation from 

 plants is simply in proportion to any one of these conditions. It is still doubtful whether 

 light, i.e. radiation as such, independently of the elevation of temperature caused 

 by it, influences transpiration ^. The stomata of most plants open more widely in light 

 than in the dark*; that is, the openings which allow of the escape of the aqueous 

 vapour formed in the interior of the tissue become larger, and this must have the effect 

 of promoting further evaporation within. It is not yet decided whether light acts on 

 the stomata as such, or by means of the heat which accompanies it, or the chemical 

 changes which it causes. 



Among the conditions connected with the organisation of the plant itself which de- 

 termine the amount of transpiration must be noticed the nature of the cortical tissue, the 

 size and number of the intercellular spaces, and the character of the substances dissolved 

 in the cell-sap. When the cortical tissue is a continuous and thick layer of periderm as 

 in many woody branches, potato-tubers, &c., or a thick layer of bark as in older trunks 

 of trees, the evaporation of water from the succulent tissues which lie beneath is rendered 



^ For further details see Nageli, Berichte der kon. bayer. Akad. 1861 ; Botanische Mittheilungen, 

 vol, I. p. 40. 



^ Sachs, Experimental-Physiologie, p. 22 r. — Miiller, Jahrb. fiir wiss. Bot. vol. VII, 1868.— 

 Baranetzky, Bot. Zeitg., 1872, Nos. 5-7.— [See also Vesque, Ann. Sci. Nat. 1877, and Burgerstein, 

 Ueb, den Einflluss aeusserer Bedingungen auf die Transpiration, Wien, 1876.] 



^ Deherain's researches (Ann. des Sci. Nat. 1869, pi. XII. p. i) do not decide the question. 



* Vou Mohl, Bot. Zeitg. 1836, p. 697. 



