BOTANY. 



Vegetable 

 Physiology. 



Perspira- 

 tion. 



Causes in- 

 ilueucmg 



Of aquatic 

 giants. 



Nature of 

 the perspi- 

 red fluid. 



them of how much importance it is to water recent- 

 ly moved plants, cuttings, grafts, and buds, in order 

 to support life. 



Leaves not only absorb, but also transpire fluids 

 to a considerable extent. This function has been cal- 

 led the perspiration of leaves, and has been consider- 

 ed similar to that of the animal economy : the 

 effects in both being evident to our senses. Hales 

 and Musschenbroeck gave the first satisfactory 

 information on this subject. The humidity that ap- 

 pears on leaves, until the time of Musschenbroeck, 

 was considered a mere condensed earthy exhala- 

 tion. This philosopher ascertained its real nature. 

 Hales extended these inquiries, and determined the 

 precise quantity of water given out by several plants. 

 His experiments were made on the vine, the annual 

 sun-flower, the cabbage, and some other plants, the de- 

 tails of which are given in almost every introductory 

 work on botany, to which we must refer. 



The state of the atmosphere influences very much 

 the rapidity of perspiration, as is well known to prac- 

 tical botanists and gardeners. Succulent plants per- 

 spire sparingly ; and evergreens, though not generally 

 succulent, yield very little moisture. The thin leaves 

 yield most perspired matter; the example of the 

 most excessive perspiration being in a tree whose 

 leaves are remarkably thin. It is the cornelian cher- 

 ry, (Corn mascula,~) which Duhamel states to throw 

 out, in the course of twenty- four hours, a quantity 

 of fluid equal to twice its own weight ; a quantity 

 almost inconceivable. 



Aquatic and bog plants seem to perspire much more 

 copiously than any others. This corresponds with 

 their absorbing powers, and is probably owing to 

 their extreme vascularity. The genus Nympkcea, and 

 the genus Potamogeton, are examples which are known 

 to every one. The genus Sarracaiia, and the Nepen- 

 thes distillatoria, are also well known to botanists, 

 and have long attracted notice from the singularity of 

 their economy, which has puzzled all that have pre- 

 tended to understand it. The leaves of the former 

 of these genera is so constructed, as effectually to 

 exclude the rain from the hollow contained by them, 

 and yet that cavity always contains a quantity of wa- 

 ter, which the plant must certainly secrete. Dr 

 Smith conjectures, that it serves as a reservoir for the 

 food of an insect of the Sphex or Ichneumon kind ; 

 and that the gas evolved from the dead flies there 

 deposited may be beneficial to the plant. This may 

 be, but we have no proof of its actually being the 

 case. The leaves of the Nepenthes distillatoriu form 

 *a close shut tube, containing an ounce or two of wa- 

 ter, which is secreted through the foot-stalk of the 

 leaf. In this cavity a small sqiiilla or shrimp, firot 

 described by Rumphius, lives and thrives, while eve- 

 ry other insect that enters it dies. The same ex- 

 planation given of the Sarracenia has been also ap- 

 plied to this plant. And it may be correct to a 

 certain extent, that is, in so far as it considers the 

 small insects as the food of the shrimp. 



The fluid perspired is said to resemble sap 

 in all its sensible properties, except in some 

 exceptions. It is probable, however, that there 

 are some important differences, although they 

 cannot be detected by our present chemical t;sts. 



This pure fluid is not the sole matter which the 

 leaves give out. A saccharine matter has been ob- 

 served on the leaves of orange trees ; a glutinous mat- 

 ter on those of the lime tree ; and a resinous matter 

 on those of the poplar, and of the Cislits creticus. 

 Manna has been scraped off the leaves of the Fraxinus 

 Omits; and a highly inflammable gas is secreted by 

 the Dictamnus dibits. Occasionally a kind of honey 

 dew is excreted by the leaves when the plant is in an 

 unhealthy state. Wax has been gathered from the 

 leaves of Rosemary. 



There is another class of operations performed by 

 the leaf, which merits and has obtained much careful 

 examination, and which has opened a field, as might 

 have been anticipated, for most brilliant specu- 

 lations. The phenomena to which we allude, are 

 those chemical effects which are induced by leaves 

 on the surrounding atmosphere ; and the influence of 

 light and heat in promoting those effects, as well as 

 some others which we shall come afterwards to notice. 

 The particular organs of the plant concerned in these 

 operations have not hitherto been completely discover- 

 ed, although much important knowledge has been at- 

 tained with regard to their effects within a very few 

 years. It is probable, however, that the powers of 

 life have full scope in these, and in all the other ve- 

 getable functions performed during its life. What- 

 ever difference of opinion exists with regard to the 

 influence of light and heat on the leaf, there can be 

 no doubt that air is indispensibly necessary for its 

 existence. Tkis fact, as well as the relative influence 

 of the leaf on the air, has been demonstrated by Hales, 

 Duhamel, Knight, and others, in the most ample 

 and satisfactory manner. It haring been once deter- 

 mined that air is required for vegetation, the next ob- 

 jects of inquiry are, what portion of the atmospherical 

 air is employed ; whether a part or the whole of it is 

 consumed ; and what are the agents engaged in con- 

 ducting the process. Philosophers have eagerly 

 sought for solutions of these inquiries ; and we find 

 the illustrious names of Scheele, Priestley, Ingen- 

 housz, Saussure junior, Senebier, Davy, Perceval, 

 Henry, and Ellis, among those that distinguished 

 themselves by prosecuting this department of vege- 

 table physiology with unwearied zeal. 



Dr Priestley made numerous experiments on the 

 action of leaves on the atmosphere ; and he drew the 

 conclusion which has been most generally adopted 

 by succeeding chemists, that they possess the power 

 of absorbing carbonic acid, and of emitting oxygen 

 gas. This doctrine was founded on the vigour with 

 which vegetation will go on under certain circum- 

 stances in carbonic acid gas, which disappears, and 

 oxygen is developed. Ingenhousz, it is generally 

 thought, has established, that the oxygen gas is 

 emitted by the leaves ; and it was inferred, that the 

 decomposition of the carbonic acid gas, in the paren- 

 chymatous substance of the leaf, furnished the emit- 

 ted gas, the whole of which, however, was not dis- 

 engaged, but a part retained. It is also stated, that 

 a small portion of azote is emitted. It is also re- 

 marked, that this absorption, decomposition, and 

 emission, take place only in the light. And it is also 

 the prevalent opinion, that the leaves of plants dur- 

 ing the day give out moisture, absorb carbonic acid 



Vegetal)! 

 Physiolog 



Other ex 

 dationt, 



Change* i 

 the air. 



Opinion! 

 Priealey 



