OZ .MOLECULAR FORCES IN THE PLANT. 



act 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 external 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 (j. 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 w^hen 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^ 



(Z*) Transpiration"^ — i.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 nf thp prrp atpr or less tendency to 

 evaporation. It must notJi £iMi u ¥Ci -tie expected th at the amount of evapo ration from 

 plants issim^ljjjj^rrrportion to ari.jLXw*e-m tnese conditions. It is still doubtTul whether 

 e. radiation as such, independently of the elevation of temperature caus^ 

 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 theti^^ct 

 .ofpromoting further evaporation within. Tli^iml jdl i h i ii1i i 1 nlli l lhi l i Lli rnl on 

 thestTTTwa ta as such, or by means -oftheTTeatwhich 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 even a thick layer of bark as in older 

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

 rendered difficult in the extreme. The cuticularised outer wall of the epidermis of 



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

 Vol. II, p. 40. 



- Sachs, Experimental-Physiologie, p. 221. — Miiller, Jahrb. fiir wiss. Bot., Vol. VII; 1868. 

 — Baranetzky, Bot. Zeit., 1872, Nos. 5-7. 



2 Deherain's recent researches (Ann. des sci. nat. 1S69, pi. XII, p. i) do not decide the question. 



* Von Mohl, Bot. Zeit. 1836, p. 697. 



