THE FARMERS' REGISTER. 



355 



carbonic acid decomposed is in proportion lo the 

 iuieiisiiy ol ilie ligiil which Blnkes a lea', ihe 

 smallesi aniouiU being iii bliady places ; and ihc 

 Ijeaiilnness ol a plant is, cceteris paribus, in pro- 

 portion 10 the quaniiiy ol carbonic acid decom- 

 posed; iherelore, ihe healthinessi ol a plant should 

 be in proportion to the quantity ol' hglu it receives 

 by day. 



6d. Uut, while this is true as a general a.xiom, 

 it IS necessary to observe that some pianis are 

 naturally inhabitants ol' shauy siiuaiions, and are 

 to organized as to be lii lor such places and lor 

 no oUiers : plants ol' this descnplion will noi 

 endure lull expoaure lo the sun ; not because an 

 abundafti decomposition ol' carbonic acid is oiher- 

 vvise than lavorable to them, but because llieir 

 epidermis allows ihe escajje ol water loo Ireely 

 by insensible pui'spiraiion, under the eolar su- 

 luulus. 



ti9. The mere lacl of plants absorbing fluids 

 from the earihj would renUer a probable that they 

 have some means ol' parling with a portion ol 

 It by their surl'dce; but that ihey do perspire is 

 Kuscej)libie ol' direct proul, and is by no means a 

 mere matter ol inlerence. 



70. We do not indeed see vapor flying ofl' from 

 the surlace ol plants ; neither do we Irom ihai 

 ol' animals, exce|)t when the air is so cold as to 

 condense the vapor ; yet we know that in both 

 cases perspiration is perpeiually going on, and it 

 would appear that in plants it lukLS place more 

 abundanUy than m animals. Il a plant covered 

 with leaves is placed under a glass vessel, and 

 exposed to the eun, the sides ol' the vessel are 

 speedily covered with dew, produced by the con- 

 densation ol' the insensible perspiration of ihe 

 plant. 11 tbe branch ol' a plant is placed in a 

 botile of water, and the neck ol" the bottle is 

 luted to the branch, so that ub evaporation can 

 lake place, nevertheless the water will disappear ; 

 and this can only happen Irom its having been 

 abstracted by the branch which lost it again 

 by insensible perspiration. Hales, an excellent 

 observer, devised many experiments connected 

 with this subject ;* among others the lollowing, 

 which he relates thus : — " August 13. In the 

 very dry year 1723, I dug down 2^ leet deep to 

 the root ol a thriving baking pear tree, and laying 

 bare a root half an inch in diameter (Jig. 7) I cul 

 ofl the end of the root at i, and put the remaining 

 stump (i n) into the glass tube dr, which was 

 an inch in diameter, and eight inches long, cemeni- 



Ihe power of absorbing by their roots, moisture, and, 

 along with it, air and carbonic acid. Is it, therefore, 

 surprising that the carbonic acid should be returned 

 unchanged to the atmosphere, along with water, when 

 light (the cause of the fixation of its carbon) is 

 absent ? Neither this emission of carbonic acid nor 

 the absorption of oxygen has any connexion with the 

 process of assimilation ; nor have they the slightest 

 relation to one another ; the one is a purely mechani- 

 cal, the other a purely chemical process. A cotton 

 wick, enclosed in a lamp which contains a liquid 

 saturated with carbonic acid, acts exactly in the same 

 manner as a living plant in the night. Water and 

 carbonic acid are sucked up by capillary attraction, 

 and both evaporate from the "exterior part of the 

 wick." Liebig, Organic Chemistry hi its applications 

 to Agriculture and Physiology, (London, 1840,) pp. 27 — j 

 33, passim ; a work which comprises a masterly view 

 of the chemical phenomena of vegetation. A. G] 

 ♦ See 'Vegetable Statics, London, 1727. I 



ing il fast al r ; the lower pari of the tube d z 

 was eighteen inches long and a quarter ol an 

 inch diameter in bore. . . . 'I'hen I turned the 

 lower end of the tube {z) uppernio.?l, and filled 

 It lull of water, and then immediately iiumeieed 

 the small end z into the cistern of meicury at the 

 bottom, taking away my linger which slopped 

 up the end ol the lube z The root im- 

 bibed the water with so much vigor, that in six 

 minutes' liine the mercury was raised uj) the tube 

 d z as high as z, namely, eight inches. . . . The 

 next morning at eight o'clock the mercury was 

 lallen to two inches in height, and two inches 

 01 the end of the root i were yet immersed in 

 water. As ihe root imbibed ihe water, innu- 

 merable air bubbles issued out at i. which oc- 

 cupied the upper part ol the tube at r as the 

 water leltit.' On another occasion he planted 

 a sunflower 3^ leet high in a garden pot, which 

 he covered with thin milled lead, cemeniing all 

 the joints so that no vapor could escape exce()l 

 through the sides of the pot and through the 

 plant itself; but providing an aperture capable 

 of being stopped, through which the earth in Ihe 

 pot could he watered. Alter tilleen uays, viz., 

 irom July 3 to August 8, he Ibund, upon making 

 all necessary allowances lor waste, that this sun- 

 flower plant 3^- leet high, with a surlace of 5616 

 square inches above the grouud, had perspired 

 as Ibllows : — 



Ounces. 

 Avoiidupoiij. 



In twelve hours of a very dry warm day 30, 

 On another day - - - - 20, 



In a dry warm night without dew - 3, 

 In a night with some small dew - U; 

 And that v\ hen the dew was copious, or there was 

 rain during the night, tlie |)lant and pot were in- 

 creased in weighl two or three ounces. Other 

 persons have insliiuied other experiments ol a 

 similar nature, the result of all which is, that the 

 insensible perspiration of plants is very conside- 

 rable.* Hales says his sunflower perspired seven- 

 teen times more than a man. There is, however, 

 this important peculiarity in vegetable perspira- 

 tion, thai it lakes place only or principally in sun- 



• The amount of this force is strikingly illustrated 

 by the following circumstances recorded by the late 

 Mr. Braddick. " One experiment I will mention, 

 as it may serve to show the great power of the rising 

 sap in tne vine, while its buds are breaking. On the 

 20th of March, in the middle of a warm day, I se- 

 lected a strong, seedling vine five years old, which 

 grew in a well prepared soil, against a south-west 

 wall ; I took off its head horizontally with a clean 

 cut, and immediately observed the sap rising rapidly 

 through all the pores of the wood, from the centre to 

 thebaik. I wiped away the exuded moisture, and 

 covered the wound with a piece of bladder, which I 

 securely fastened with cement, and a strong binding 

 of waxed twine. The bladder, although first drawn 

 very close to the top of the shoot, soon began to 

 stretch, and to rise like a ball over the wound; thus 

 distended, and filled with the sap of the vine, it felt 

 as hard as a cricket ball; and seemed, to all appear- 

 ance, as if it would burst. I caused cold water from 

 a vyell to be thrown on the roots of the plant ; but 

 neither this nor any other plan that I could devise, 

 prevented the sap from flowing, which it continued 

 to do with so much force as to burst the bladder in 

 about forty-eight hours after the operation was per- 

 formed ; the weather continuing the whole time warm 

 and genial.— ifprt. Tra?is., v. 202.) 



