620 London Hurticultural Society/ and Garden. 



capillary attraction ; others to the alternate expansion and contrac- 

 tion of the medullary rays, from hyf^rometrical changes; others to fer- 

 mentation ; and otliers to the action of the leaf l)uds. This last appears 

 the most prohahle ; the ex[)anding leaves, by constantly consuming the 

 sap nearest to them, create a vacuum, which is as constantly supi)lied by 

 the fresh sap ascending from below ; and this theory accounts for the 

 motion of the sap being always most apparent at the extremity of the 

 branches, when they first begin to expand themselves into leaves. The 

 only difficulty is, to know how this action is primarily produced. It has 

 l)een observed that the fluid contained in the leaf buds is nnich denser 

 than that in the body of the tree ; and Dutrochet, a French physiologist, 

 having discovered, by a scries of experiments (Vol. III. p. 78.), that dense 

 fluids possess an attraction for lighter ones, supposed that in this manner 

 he could account for the circulation of the sap. This jjiinciple he called 

 the endosmose, or inward impulse; its application, however, to the circu- 

 lation of the sap, has, it appears, been since abandoned by himself as un- 

 tenable (Vol. VII. p. 59.); and we are driven back to the supposition 

 that it depends upon vital irritability, and is independent of mechanical 

 causes. 



Carbon has been proved to be essential to the existence of plants ; and 

 it appears probable that their vigour depends upon the quantity of carbonic 

 acid which they are enabled to absorb. In the earlier ages of the world, car- 

 bonic acid is supposed to have existed to such an extent in the atmosphere, 

 as to render it unfit for the respiration of animals. At this jjeriod, as we may 

 judge from fossil organic remains, plants acijuired extriiordinary size luid 

 strength ; but, from the extreme rarity of floral specimens in the remains 

 alluded to, they do not seem to have produced either flowers or fruits, 

 except in some few instances. It has been already shown that flowers 

 and fruit are only stunted branches, produced by accumulations of sap, or, 

 in fact, by partial disease, or imperfection in the circulation ; the abun- 

 dant siipply of carbonic acid, which occasioned the sap to flow freely and 

 copiously, was therefore inimical to their formation. The carbonic acid 

 absorbeci by plants during this early period being decomposed, and the 

 ox\gen which it contained being again given out into the atmosphere, the 

 portion of oxygen in the air became greater, and the plants being deprived 

 of their too luxuriant food began to produce flowers and fruit ; and 

 then appeared cold-blooded animals, which are able to breathe air nuich 

 more impure than warm-blooded ones. A further purification of the 

 uir took i)lace, and finally it was rendered fit for the respiration of man 

 iuid other warm-bloodcil animals possessing hearts : plants thus, by the 

 bountiful provision of nature, makiug the air more fit for man. The fact 

 tiiat preternatural accumulations of sap dispose plants to bear flowers and 

 fruit, is proved by the well-known practice of rniging a sterile tree : this 

 produces an accumulation of sap above the wound ; and the free 

 circulation of the sap being thus im|)eded, it is forced to expand itself 

 in fruit. The extreme heat of last sinnmer caused extraordinary 

 evaporation, and the density of the sap being thus increased, it has 

 flowed this sjjring with less freedom than usual, and a greater abun- 

 dance of flowers has been the result. An expedient used to make the 

 jujube tree (Zizyphus Jujitha Lamarck) at Malta produce more abundantly, 

 may be accounted for on the same princi()le. A large piece of limestone 

 iii |)ut between the forketl branches of the tree, which by its weight bends 

 and contracts the sap-vessels, and prevents the free circulation of the sap. 

 When speaking of the descending saj). Professor Lindley alluded to some 

 e\|)eriments tried by Mr. Jose|)li Thom|)son of Welbeck (detailed Vol. V. 

 |). -ili'-i. 2.')7.). lie observed, also, that plants iuibii)ed oxygen during the 

 night, and gave it out again during the day; and he showed a diagram, 



