London Horticultural Society and Garden. 6 1 9 



similar nature. Thus, a decoction of A'tropa Belladonna L. was found to 

 kill a plant of Belladonna, &c. The substances thrown off' by the root are 

 not the only secretions trees have the power of making. Some trees 

 secrete gum, &c, from the sap in its progress downwards, which they dis- 

 charge from the bark. Mineral poisons do not operate upon plants, any 

 more than upon animals, in a crude state. Mercury requires to be oxi- 

 dised, &c. 



Notwithstanding the bad effects produced upon plants by injurious 

 substances being mingled with the aqueous particles they imbibe, they will 

 not thrive well in pure water ; and the experiments which have appeared 

 to prove that they can exist on water alone have always, on examination, 

 been found to be fallacious. Sap is composed chiefly of water, mucilage, 

 and sugar ,• but it does not attain its proper qualities till it has been ex- 

 posed to the action of the atmospheric ah- in the leaves : it rises through 

 the alburnum, in that part nearest the extremity of the medullary rays ; 

 but it is never seen in the heart wood, the vessels once destined for its 

 transmission being found choked up in the centre of the tree when it has 

 attained that state. The sap, having reached the leaves, is there exposed 

 to the action of the atmospheric air, much in the same way as the blood is in 

 the lungs, with, however, this important difference, that, while animal blood 

 principally absorbs oxygen and gives out carbonic acid gas, &c, sap appears 

 to have the greatest afnnityfor carbonic acid, and to be compelled to give out 

 a portion of its oxygen before it is fit to afford proper nourishment to the 

 plant. The sap first spreads itself along the upper surface of the leaves, 

 immediately under the membrane of the cuticle, and flows back along 

 the lower stratum of veins, in the under side of the leaves. As soon as 

 its elaboration is complete, it descends by the bark, radiating, in its pro- 

 gress, towards the centre of the tree, by means of the silver grain or me- 

 dullary rays, which consist of masses of cellular tissue, exquisitely though 

 invisibly permeable to fluids. As the sap, during the process of its ela- 

 boration in the leaves, has not only to imbibe certain qualities, but to 

 throw off others, plants, like animals, are provided with the necessary 

 organs for a double kind of respiration : these functions, however, are 

 never perfectly performed without the assistance of light. When a plant 

 is exposed to the full action of the sun's rays, it gives out pure oxygen, 

 and absorbs carbonic acid, which is afterwards decomposed in the plant, 

 the carbon only being retained. This operation cannot go on in constant 

 darkness, the plant then only giving out carbonic acid and nitrogen 

 gases, without oxygen, and becoming sickly and languishing. Hence 

 arise the efforts which plants placed in obscure situations are con- 

 tinually making to reach the light. The evaporation of plants is seventeen 

 times greater than that of animals ; they consequently require a con- 

 stant supply of aqueous particles at their roots, to support this copious 

 evaporation, and when deprived of this they perish. In warm dry wea- 

 ther the evaporation is of course greater than in cold and moist weather ; 

 and it is always greatest when the leaves are expanded. In winter the 

 evaporation is very trifling ; and the reason that transplanting is less in- 

 jurious in that season than in summer is, that the spongioles, being injured 

 by transplanting, are, in hot dry weather, unable to imbibe sufficient mois- 

 ture to support the excessive evaporation which takes place through the 

 leaves. As an illustration of this remark, it may be observed that plants 

 in pots, not being liable to have their spongioles injured, will bear removal 

 at any season with perfect ease. 



Many hypotheses have been started with regard to the cause of the cir- 

 culation of the sap. Some have supposed the sap to rise, from its alter- 

 nate rarefaction and condensation by the changes of temperature ; others 

 that the sap-vessels are furnished with valves; others attribute it to 



