Oct. 24, 1878] 



NATURE 



6/3 



Bphere containing carbonic acid, decomposed i6c.c. of 

 this gas ; one containing 36 per cent, decomposed 1 1 c.c. ; 

 |ind one containing but 29 per cent, was without action. 



A series of observations was made on the relative 

 |30wers of evapot ation on the upper and lower sides of 

 leaves. They consisted in plucking two leaves of the 

 pame kind at the same moment, covering on the one the 

 upper, on the other the lower side with melted tallow, 

 and then noticing the loss of weight by evaporation in a 

 given time. The average of the results showed that the 

 proportion between the amounts of water evaporated on 

 the upper and lower side of a dozen varieties of leaves 

 was I ; 4'3. In all cases the amount evaporated from the 

 two exposed sides of two equal leaves was greater than 

 from the entire surface of a similar leaf under the same 

 circumstances. 



A point of no small interest with regard to the physical 

 function of leaves is that of their ability to replace the 

 roots of a plant in serving as the agent of absorption. A 

 variety of tests were undertaken to settle this question ; 

 among them the following : — A forked branch of lilac 

 (Fig. i) was so disposed that the one branch was im- 

 mersed in water while the other was exposed to the 

 ordinary atmospheric conditions. The superficies of 

 foliage was the same on both branches. The transpira- 



FlG. 1. 



tion from the surface of the leaves on the latter branch 

 was the same as under normal circumstances, and after 

 the lapse of two weeks the foliage was as fresh as at the 

 commencement, showing that the submerged leaves were 

 fully able to replace the roots in one of their functions. 

 In an experiment with a beet in which one-half of the 

 leaves were in water and one-half in the air, communica- 

 tion being maintained by means of the root, the free por- 

 tion of the leaves wilted in the course of a day, the neck 

 of the root apparently not offering a sufficient means of 

 communication with the submerged leaves. A grape- 

 vine shoot half plunged in water (Fig. 2) maintained a 

 normal evaporation in the free foliage, and remained 

 fresh for over a month. An oleander shoot under similar 

 conditions maintained its normal appearance for four 

 months. With the artichoke it was found necessary that 

 the surface of the leaves beneath the water should be 

 four times that of the leaves above. 



Closely bordering on this question is another which 

 has excited much dispute, viz., the ability of leaves to 

 draw water from the surrounding air or by immersion, 

 after having suffered losses by transpiration. Prof. 

 Boussingault' s numerous experiments show that leaves. 



after having been exposed to influences causing a rapid 

 evaporation, are able to absorb water rapidly on immer- 

 sion, and even from an atmosphere saturated with 

 aqueous vapour. There is, however, in both cases no 

 absorption unless the leaves have lost a portion of their 

 water of constitution, i.e., that which is essential to their 

 normal existence. Thus, a wilted branch of periwinkle 

 weighing 4*0 grammes, after remaining in an atmosphere 

 saturated with aqueous vapour for a day and a half, 

 weighed 4*2 grammes, and after twelve hours' immersion 

 in water 9*4 grammes. 

 The last function of leaves studied by Prof. Bous- 



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singault is their ability to absorb solutions of mineral 

 matter, i.e., perform another of the ordinary duties of the 

 roots. For this purpose a solution of gypsum containing 

 Tt^<T of solid matter was used. Drops of this solution 

 were placed on the leaves of a great variety of plants — 

 under conditions favouring absorption, as in the experi- 

 ments just described — and protected from evaporation 

 by superincumbent watch-glasses with greased edges 



(fig. 3)- 



In most instances the drops were absorbed entirely, 

 leaving no traces of the mineral matter ; in some cases 

 a slight residue was left, which the addition of a minute 



Fig. 3- — a, drop of solution ; u, watcb glass. 



quantity of water caused to disappear. As in the case of 

 pure water, the under side of the leaves absorbed much 

 more rapidly than the upper side. Solutions of sulphate 

 and nitrate of potassium gave quite similar results ; the 

 absorption of solutions of chloride of sodium and nitrate 

 of ammonium was not so perfect. These results would 

 tend to show that the foliage of a plant is able to supply 

 it with perhaps no small portion of its sahne consti- 

 tuents by means of the ammoniacal salts formed in the 

 air, and the alkaline and earthy salts suspended there 

 which are deposited on the surface of the leaves by rain 

 and dew. T. H. N. J 



