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 Nägeli, Berichte der kön. bayer, Akad. iSöi ; Botanische Mittheilungen, 
vol. I. p. 40. 
^ Sachs, Experimental-Physiologie, p. 221. — Müller, Jahrb. für wiss. Bot. vol. VII, 186S. — 
Baranetzky, Bot. Zeitg., 1872, Nos. 5-7.— [See also Vesque, Ann. Sei. Nat. 1877, and Burgerstein, 
Ueb. den Einfluss aeusserer Bedingungen auf die Transpiration, Wien, 1876.] 
2 Deherain's researches (Ann. des Sei. Nat. 1869, pl. XII. p. i) do not decide the question. 
* Von Mold, Bot. Zeitg. 1836, p. 697. 
