170 SCIENCE OF GARDENING. ! Part II. 



the summer on the leaves of the maple, poplar, and lime-tree; but particularly on the 

 surface exposed to the sun, which it sometimes wholly covers. 



Its physical as well as chemical qualities are very different in different species of plants ; so that it is not 

 always merely an exudation of sap, but of sap in a high state of elaboration, or mingled with the peculiar 

 juices or secretions of the plant. Sometimes it is a clear and watery fluid conglomerating into large drops, 

 such as are said to have been observed by Miller, exuding from the leaves of the musa arbor, or plantain, 

 tree ; and such as are sometimes to be seen in hot and calm weather exuding from the leaves of the poplar 

 or willow, and trickling down in such abundance as to resemble a slight shower. This phenomenon was 

 observed by Sir J. E. Smith, under a grove of willows in Italy, and is said to occur sometimes even in Eng- . 

 land. Sometimes 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 redundancy of the fluid thrown off by imper- 

 ceptible 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 cater- 

 pillar 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. 



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

 the 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. 



756. 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 effected ? 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. Priestley 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; andSaussure, that the elaboration 

 of carbonic acid gas is essential to vegetation in the sun ; and, finaily, 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 effect 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. 



757. Elaboration of oxygen. It has been already 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- 

 veyit 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 plant 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 effect also 

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

 of such union. But 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 bemuse 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 

 turtner 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, excent by increasing 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, suffer 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 



