ABSORPTION. 



ABSORPTION. 



capacity of absorbing moisture chiefly for the tissue, compact, spongy, and the whole newly 

 purpose of catching the ascending exhalations developed, possessing in a high degree the 

 which must necessarily come in contact with hygroscopical faculty proper to vegetable 

 it as they rise, but which might possibly have | tissue. 



escaped if absorbable only by the upper sur- M. Carradori (Degli Organi Assorbtnti) has 

 face, owing to the increased rapidity of their 

 ascent at an increased elevation ; and regard- 

 ing the upper surface of the leaf of the herb as 

 being endowed with the greater absorbing 



power, owing to its low stature and the slow 

 ascent of exhalations near the earth. This did 

 not throw much light upon the subject ; and 

 the experiments were still deemed insufficient, 

 as not representing to us the actual pheno- 

 menon of vegetation, though the fact of the 

 absorption of moisture by the leaf is fully 

 confirmed. 



If, after a long drought, a fog happens to 

 succeed before any rain falls, so as to moisten 

 the surface of the leaves, plants begin to re- 

 vive, and to resume their verdure long before 

 any moisture can have penetrated to their 

 roots. Hence it follows incontestibly, either 

 that moisture has been absorbed by the leaf, or 

 that exhalation has been suddenly stopped by 

 closing the pores of the leaf, or both. The ef- 

 ficacy of rain and of artificial waterings may 

 be accounted for partly on the same principle ; 

 for they have not always penetrated to the root 

 when they are found to have given freshness 

 to the plant by either or both of the processes 

 just alluded to. The moisture, then, that enters 

 the plant as an aliment, is taken up by means 

 of the pores; or, in default of visible pores, 

 merely by means of the absorbent power of the 

 outer bark (epidermis), not only of the root 

 and leaf, but often, as it is to be believed, of 

 the other parts of the plant also, at least when 

 they are in a soft and succulent state. 



It is to the modern improvements in pneu- 

 matic chemistry, and to them alone, that we 

 are indebted for our knowledge of the real 

 functions of the leaves of plants, and of their 

 analogical resemblance to the lungs of animals, 

 it being now proved indisputably that the 

 leaves of plants -not only contain air, but do 

 both inhale and respire it. It was the opinion 

 of Dr. Priestley that they inhale it chiefly by 

 the upper surface ; and it has been shown by 

 Saussure that their inhaling power depends 

 entirely upon the integrity of their organisa- 

 tion. A bough of Cactus Opuntia, detached 

 from the plant and placed in an atmosphere 

 of common air, inhaled in the course of a 

 night four cubic inches of oxygen ; but when 

 placed in a similar atmosphere, after being cut 

 to pieces and pounded in a mortar, no inhala 

 tion took place. The inhalation of air, there- 

 fore, is no doubt effected by the pores of the 

 outer bark (epidermis} of the leaf. 



It is important to attend particularly to the 

 distinction pointed out above, that it is' not the 

 whole of the root which is endowed with the 

 power of absorbing nourishment, but only the 

 points of the root fibres, termed spongelets. 

 The surface of the root whose outer bark has 

 acquired a certain consistence does not absorb 

 the moisture of the soil in contact with it ; bu 

 the roots, and also the smallest rootlets, con- 

 stantly lengthen at their extremities ; and these 

 extremities are composed of a fine cellular 



remarked that there is a slight absorption, 

 either by the surface of the roots, or by the 

 ugacious hairs with which the roots of young 

 jlants are often furnished : but this effect 

 seems owing to general hygroscopicity; and 

 le himself agrees that this absorption is ex- 

 remely feeble, especially in old and woody 

 roots, comparatively with that which takes 

 alace at their extremities. These experiments, 

 lowever, are not made with such minute accu- 

 racy as to enable us to appreciate this com- 

 parison. 



When we cut a branch of a tree and plunge 

 it into water, its woody tissue thus laid bare 

 quickly absorbs a quantity of water ; and in 

 this manner is the life of branches preserved 

 which are kept for ornamental purposes, but 

 this effect has a limit. The extremity which 

 das been cut and plunged in the water is not 

 renewed, as in the case of the root; and is, 

 consequently, more or less quickly altered or 

 deteriorated by being in contact with the water. 

 We renew its action by cutting off the rotting 

 extremity, and thus place a new and healthy 

 surface in contact with the liquid. The water 

 which in this manner penetrates into the 

 woody tissue of vegetables, preserves their ex- 

 istence, at least for a certain time, as if it en- 

 tered by the spongelets. This is the same 

 thing, we may rest assured, in these pheno- 

 mena, as is presented in the developement of 

 the cuttings of trees, which are also nourished 

 in general only through the water sucked up 

 by the surface of their denuded wood. These 

 means of nutrition are, however, accidental or 

 artificial ; and absorption is a natural opera- 

 tion by the spongelets in general, or by the 

 suckers in some vegetable parasites. M. Sen- 

 nebier observed that, if we divide a plant into 

 three parts, the roots as far as the crown, the 

 stem as far as the branches, and the leafy top, 

 then plunge the lower ends of these into water, 

 the whole three will pump up a certain quan- 

 tity, but the leafy parts more than the others. 

 This absorption particularly takes place at the 

 cut surface, where the woody parts are laid 

 bare. 



A branch of raspberry put in water and ex 

 posed to the sun has absorbed a hundred and 

 fifty grains, but only imbibed eight grains when 

 the division has been covered over with wax. 

 It sucked up no more when, having the divided 

 part covered, it was plunged in the whole of its 

 length, than when only a short zone at the ex- 

 tremity was immersed. This proves that the 

 outer bark is impenetrable to water. 



The woody portion, when laid bare, sucks 

 up moisture in every way ; that, is to say, when 

 we cut a branch and place it in the water, it 

 sucks it up, either when put into it by the 

 upper or by the lower cut part. The habitual 

 or upright direction, however, appears to offer 

 certain facilities for this more than an inverse 

 one. This, indeed, results, first, from the ob- 

 servation of M, Pollini (Elem. di Botan., i. 281) ; 

 for the watery juices mount a little less high 



B 



13 



