1S38] 



FARMERS' REGISTER. 



635 



Ilalcp, vol. i. of the Vegci.able Statics, page 114: 

 A vine branch, four or five years old, was cut 

 llirough, and a glass tube carefully attached to it ; 

 this lube was bent as a syphon, and filled with 

 quicksilver ; so that the force ol' liie ascending sap 

 could be measured by its effect in elevating the 

 quicksilver. In a few days it was found that the 

 sap had been propelled forwards with so much 

 force as to raise the quicksilver to 38 inches, 

 which is a force considerably superior to that of 

 the usual pressure of the atmosphere. Capillary 

 attraction can only be exerted by the surfaces of 

 small vessels, and can never raise a fluid into 

 tubes above the vessels themselves. 



I referred, in the beginning of the third lecture, 

 to Mr. Knight's opinion, that the contractions 

 ■and expansions of the silver grain in the alburnum 

 are the most efficient cause of the ascent of the 

 fluids contained in its pores and vessels. The 

 views of this excellent physiologist are rendered 

 extremely probable by the facts he has brought 

 forward in support of them. Mr. Knight found 

 that a very small increase of temperature was 

 sufficient to cause the fibres of the silver grain to 

 separate from each other, and that a very slight 

 diminution of heat produced their contraction. 

 The sap rises most vigorously in spring and au- 

 tumn, at the time the temperature is variable ; and 

 if it be supposed that, in expanding and contract- 

 ing, the elastic fibres of the silver grain exercise a 

 pressure upon the cells and tubes containing the 

 fluid absorbed by the capillary attraction of the 

 roots, this fluid must constantly move upwards to- 

 wards the points where a supply is needed. 



The experiments of Montsiolfier, the celebrated 

 inventor of the balloon, have shown that water 

 may be raised almost to an indefinite height by a 

 very small force, provided its pressure be taken off 

 by continued divisions in the column of fluid. 

 This principle, there is great reason to suppose, 

 must operate in assisting the ascent of the sap in 

 the cells and vessels of plants which have no rec- 

 tilineal communication, and which every where 

 oppose obstacles to the perpendicular pressure of 

 the sap. 



The changes taking place in the leaves and 

 buds, and ihe decree of their power of transpira- 

 tion, must be intimately connected likewise with 

 the motion of the sap upwards. This is shown 

 by several experiments of Dr. Hales. 



A branch from an apple-tree was separated and 

 introduced mto Vi'ater, and connected with a mer- 

 curial gauge. When the leaves were upon it, it 

 raised the mercury by the force of the ascending 

 juices to four inches; but a similar branch, from 

 which the leaves were removed, scarcely raised it 

 a quarter of an inch. 



Those trees, likewise, whose leaves are soft and 

 of a spongy texture, and porous at their upper 

 surfaces, displayed by far the greatest powers with 

 regard to the elevation of the sap. 



The same accurate philosopher whom I have 

 just quoted found that the pear, quince, cherry, 

 walnut, peach, gooseberry, water-elder, and syca- 

 more, which have all soft and unvarnished leaves, 

 raised the mercury under favorable circumstances 

 ji-om three to six inches. Whereas the elm, oak, 

 chestnut, hazel, sallow, and ash. which have firm- 

 er and more glossy leaves, raised the mercury only 

 from one to two inches. And the evergreens, and 

 trees bszaring varnished leaves, scarcely at all af- 



fected it ; parlicularly the laurel and the laurus- 

 tinus. 



It will be proper to mention the facts which 

 show that in many caso' fluids descend through 

 the bark. Mr. Knight has shown, in the Philo- 

 sophical Transactions, that long strips of hark, 

 every where detached from the alburnum of the 

 tree, except at their upper ends, deposited as 

 much alburnum as they could have done, if they 

 had retained their natural position. In these 

 cases, Ihe sap must have descended through the 

 bark wholly. 



M. Baisse placed branches of different trees in 

 an infusion of aiadder, and kept them there for a 

 long time. He found, in all cases, that the wood 

 became red before the bark; and that the bark 

 began to receive no tinsre till the whole of the 

 wood was colored, and till the leaves were affect- 

 ed ; and that the coloring matter first appeared 

 above, in the bark immediately in contact with the 

 leaves. 



Similar experiments were made by M. Bonnet, 

 and with analogous results, though not so perfectly 

 distinct as those of M. Baisse. 



Du Haniel found, that in different species of 

 the pine and other trees, when strips of bark were 

 removed, the upper part of the wound only emitted 

 fluid, whilst the lower part remained dry. 



This may likewise be observed in the summer 

 in fruit trees, when the bark is wounded, the al- 

 burnum remaining untouched. 



The motion of the sap through ihe bark seems 

 principally to depend upon gravitation. When 

 the watery particles have been considerably dissi- 

 pated by the transpiring flinclions of the leaves, 

 and the nmcilaginous, inflammable, and astrin- 

 gent constituents, increased by theagency of heat, 

 liL'ht, and air, the continued impulse upwards 

 from Ihe alburnum forces the remaining inspis- 

 sated fluid into the cortical vessels, which receive 

 no other supply. In these, from its weight, its 

 natural tendency must be to descend; and the ra- 

 pidity of the descent must depend upon the gene- 

 ral consumption of the fluids of the bark in the 

 living processes of vegetation; for there is every 

 reason to believe that no fluid passes into the soil 

 through the roots; and it is impossible to conceive 

 a free lateral communication between the absorb- 

 ent vessets of the alburnum in the roots, and the 

 transporting or carrying vessels of the bark; for if 

 such a communication existed, there is no reason 

 why the sap should not rise through the bark as 

 well as through the alburnum; for ?he same phy- 

 sical powers would then operate upon both. 



Some authors have supposed that the sap rises 

 in the alburnum, and descends through the bark, 

 in consequence of a power similar to that which 

 produces the circulation of the blood in animals; a 

 force analogous to the muscular force in the sides 

 of the vessels. 



This analogy has, however, in general, been 

 too much insisted upon and too loosely stated; 

 there are undoubtedly resemblances more or less 

 remote in ever}^ part of created nature; but the 

 irritability of the uuscular fibre in animals and the 

 contractibiiity of the vascular system in plants 

 appear to depend upon entirely different causes. 

 ' In crystallizaiion, or the regular arrangement of 

 inorganic substances, there is a constant increase 

 of matter from the attraction and juxta-position of 

 like parts or moleculet. In vegetation a germ ex- 



