620 London Horticultural Society and Garden. 



capillary attraction ; others to the alternate expansion and contrac- 

 tion of the medullary rays, from hygrometrical changes; others to fer- 

 mentation ; and others to the action of the leaf buds. This last appears 

 the most probable ; the expanding leaves, by constantly consuming the 

 sap nearest to them, create a vacuum, which is as constantly supplied 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 

 been observed that the fluid contained in the leaf buds is much denser 

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

 having discovered, by a series 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 principle 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. Vll. 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 period, as we may 

 judge from fossil organic remains, plants acquired extraordinary size and 

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

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

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

 oxygen which it contained being again given out into the atmosphere, the 

 portion of ox3'gen 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 much 

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

 air took place, and finally it was rendered fit for the respiration of man 

 and other warm-blooded animals possessing hearts : plants thus, by the 

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

 that preternatural accumulations of sap dispose plants to bear flowers and 

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

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

 circulation of the sap being thus impeded, it is forced to expand itself 

 in fruit. The extreme heat of last summer caused extraordinary 

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

 flowed this spring 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 Jujuba Lamarck) at Malta produce more abundantly, 

 may be accounted for on the same principle. A large piece of limestone 

 is put between the forked 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 sap, Professor Lindley alluded to some 

 experiments tried by Mr. Joseph Thompson of Welbeck (detailed Vol. V. 

 p. 253. 257.). He observed, also, that plants imbibed oxygen during the 

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



