238 SCIENCE OF AGRICULTURE. Part IL 



said to occuf sometimefi even In England. Bometimee it is glutinous, as on the leaf of the lime-tree ; 

 sometimes it is waxy, as on the leaves of rosemary ; sometimes it is saccharine, as on the orange-leaf; or 

 resinous, as on the leaves of the cistus creticus. The cause of this excess of perspiration has not yet been 

 altogether satisfactorily ascertained ; though it seems to be merely an effort and institution of nature to 

 throw off" all such redundant juices as may have been absorbed, or secretions as may have been formed 

 beyond what are necessary to the due nourishment or composition of the plant, or beyond what the plant 

 is capable of assimilating at the time. Hence the watery exudation is perhaps nothing more than a re- 

 dundancy of the fluid thrown oft" by imperceptible perspiration, and the waxy and resinous exudations 

 nothing more than a redundancy of secreted juices ; all which may be still perfectly consistent with a 

 healthy state of the plant. But there are cases in which the exudation is to be regarded as an indication 

 of disease, particularly in that of the exudation known by the name of honey-dew, a sweet and viscid 

 substance covering the leaves like a varnish, and sometimes occasioning their decay. Such at least seems 

 to be the fact with regard to the honey-dew of the hop, which, according to the observations of Linnasus, 

 is the consequence of the attacks of the caterpillar of the ghost-moth injuring the root. And such seems 

 also to be the fact with regard to the honey-dew of the beech-tree, and perhaps also the honey-dew of the 

 oak. The sap then in the progress of its ascent from the extremity of the root to the extremity of the 

 leaf undergoes a considerable change, first in its mixing with the juices already contained in the plant, 

 and then in its throwing off" a portion at the leaf. 



1529. The sap is further affected by means of the gases entering into the root along with 

 tiie moisture of the soil, but certainly, by means of the gases inhaled into the leaf; the 

 action and elaboration of which shall now be elucidated. 



1530. Elaboration of carbonic acid. The utility of carbonic acid gas as a vegetable food has been al- 

 ready shown ; plants being found not only to absorb it by the root along with the moisture of the soil, but 

 also to inhale it by the leaves, at least when vegetating in the sun or during the day. But how is the ela- 

 boration of this gas eff"ected ? Is it assimilated to the vegetable substance immediately upon entering the 

 plant, or is its assimilation effected by means of intermediate steps ? The gas thus inhaled or absorbed is 

 not assimilated immediately, or at least not wholly : for it is known that plants do also evolve carbonic 

 acid gas when vegetating in the shade, or during the night. Priestlsy ascertained that plants vegetating in 

 confined atmospheres evolve carbonic acid gas in the shade, or during the night, and that the vitiated 

 state of their atmospheres after experiment is owing to that evolution ; and Saussure that the elaboration 

 of carbonic acid gas is essential to vegetation in the sun ; and, finally, Senebier and Saussure proved that 

 the carbonic acid gas contained in water is abstracted and inhaled by the leaf, and immediately decom- 

 posed ; the carbon being assimilated to the substance of the plant, and the oxygen in part evolved, and 

 in part also assimilated. The decomposition of carbonic acid gas takes place only during the light of day, 

 though Saussure has made it also probable that plants decompose a part of the carbonic acid gas which 

 they form with the surrounding oxygen even in the dark. But the eff"ect is operated chiefly by means of 

 the leaves and other green parts of vegetables, that is, chiefly by the parenchyma ; the wood, roots, petals, 

 and leaves that have lost their green color not being found to exhale oxygene gas. It may be observed, 

 however, that the green color is not an absolutely essential character of the parts decomposing carbonic 

 acid ; because the leaves of a peculiar variety of the atriplex hortensis, in which all the green parts change 

 to red, do still exhale oxygene gas. 



1531. Elaboration of oxygen. It has been alreadj^ shown that the leaves of plants abstract oxygen from 

 confined atmospheres, at least when placed in the shade, though they do not inhale all the oxygen that 

 disappears ; and it has been further proved, from experiment, that the leaves of plants do also evolve' a 

 gas in the sun. From a great variety of experiments relative to the action and influence of oxygen on the 

 plant, and the contrary, the following is the sum of the results. The green parts of plants, but especially 

 the leaves, when exposed in atmospheric air to the successive influence of the light and shade, inhale and 

 evolve alternately a portion of oxygene gas mixed with carbonic acid. But the oxygen is not immediately 

 assimilated to the vegetable substance ; it is first converted into carbonic acid by means of combining with 

 the carbon of the plant, which withers if this process is prevented by the application of lime or potass. 

 The leaves of aquatics, succulent plants, and evergreens consume, in equal circumstances, less oxygen 

 than the leaves of other plants. The roots, wood, and petals, and in short all parts not green, with the 

 exception of some colored leaves, do not effect the successive and alternate inhalation and extrication of 

 oxygen ; they inhale it indeed, though they do not again give it out, or assimilate it immediately, but con- 

 vey it under the form of carbonic acid to the leaves, where it is decomposed. Oxygen is indeed assimilated 

 to the plant, but not directly, and only by means of the decomposition of carbonic acid ; when part of it, 

 though in a very small proportion, is retained also and assimilated along with the carbon. Hence the most 

 obvious influence of oxygen, as applied to the leaves, is that of forming carbonic acid gas, and thus pre- 

 senting to the plants elements which it may assimilate ; and perhaps the carbon of the extractive juices 

 absorbed even by the root, is not assimilated to the plant till it is converted by means of oxygen into car- 

 bonic acid. But as an atmosphere composed of nitrogen and carbonic acid gas only is not favorable to 

 vegetation, it is probable that oxygen performs also some other function beyond that of merely presenting 

 to the plant, under the modification of carbonic acid, elements which it may assimilate. It may eff'ect also 

 the disengagement of caloric by its union with the carbon of the vegetable, which is the necessary result 

 of such union. I^ut oxygen is also beneficial to the plant from its action on the soil ; for when the ex- 

 tractive juices contained in the soil have become exhausted, the oxygen of the atmosphere, by penetrating 

 into the earth and abstracting from it a portion of its carbon, forms a new extract to replace the first. 

 Hence we may account for a number of facts observed by the earlier phytologists, but not well explained. 

 Du Hamel remarked that the lateral roots of plants are always the more vigorous the nearer they are 

 to the surface ; but it now appears that they are the most vigorous at the surface.because they have there 

 the easiest access to the oxygen of the atmosphere, or to the extract which it may form. It was observed, 

 also, by the same phytologist, that perpendicular roots do not thrive so well, other circumstances being the 

 same, in a stiff" and wet soil as in a friable and dry soil ; while plants with slender and divided roots thrive 

 equally well in both : but this is no doubt owing to the obstacles that present themselves to the passage of 

 the oxygen in the former case, on account of the greater depth and smaller surface of the root. It was 

 further observed, that roots which penetrate into dung or into pipes conducting water, divide into immense 

 numbers of fibres, and form what is called the fox-tail root ; but it is because they cannot continue to ve- 

 getate, except by encreasing their points of contact, with the small quantity of oxygen found in such 

 mediums. Lastly, it was observed that plants, whose roots are suddenly overflowed with water remaining 

 afterwards stagnant, suff"er sooner than if the accident had happened by means of a continued current. It 

 is because in the former case the oxygen contained in the water is soon exhausted, while in the latter it is 

 not exhausted at all. And hence also we may account for the phenomenon exhibited by plants vegetating 

 in distilled water under a receiver filled with atmospheric air, which having no proper soil to supply the 

 root with nourishment, eff"ect the developement of their parts only at the expense of their own proper 

 substance ; the interior of the stem, or a portion of the root, or the lower leaves decaying and giving up 

 their extractive juices to the other parts. Thus it appears that oxygene gas, or that constituent part of 

 the atmospheric air which has been found to be indispensable to the life of animals, is also indispensable to 

 the life of vegetables. Butalthoughthepresence and action of oxygen is absolutely necessary to the process 

 of vegetation, plants do not thrive so well in an atmosphere of pure oxygen, as in an atmosphere of pure or 

 common air. This was proved by an experiment of Saussure's, who having introduced some plants of 

 pisum sativum, that were but just issuing from the seed, into a receiver containing pure oxygene gas. 



