Book I. PROCESS OF VEGETABLE NUTRITION. 237 



but to protrude their blossoms and mature their fruit, even in the midst of winter, when the temperature 

 is ,:t the lowest; and, in the case of submarine plants, the temperature can never be very high : so 

 that, although heat does no doubt facilitate the ascent of the sap by its tendency to make the vessels 

 expand, yet it cannot be regarded as the efficient cause, since the sap is proved to be in motion even 

 throughout the whole of the winter. I)u Hamel endeavours, however, to strengthen the operation of 

 heat by means of the influence of humidity, as being also powerful in promoting the ascent of the >ap, 

 whether as relative to the season of the year or time of the day. The influence of the humidity of the 

 atmosphere cannot be conceived to operate as a propelling cause, though it may easily be conceived to 

 operate as affording a facility to the ascent of the sap in one way or other; which under certain circum- 

 stances is capable of most extraordinary acceleration, but particularly in that state of the atmosphere 

 which forbodes or precedes a storm. In such a state a stalk of wheat was observed by Du Hamel to grow 

 three inches in three days ; a stalk of barley six inches, and a shoot of a vine almost two feet ; but this 

 is a state that occurs but seldom, and cannot be of much service in the general propulsion of the sap. 

 On til is intricate but important subject Linnaeus appears to have embraced the opinion of Du Hamel, or 

 an opinion very nearly allied to it ; but does not seem to have strengthened it by any new accession of 

 argument ; so that none of the hitherto alleged causes can be regarded as adequate to the production of 

 the effect 



1547. Irritability. Perhaps the only adequate cause ever suggested, prior to the hypothesis of 

 Dutrochet, is that alleged by Saussure. According to Saussure the cause of the sap's ascent is to 

 be found in a peculiar species' of irritability inherent in the sap-vessels themselves, and dependent upon 

 vegetable life ; in consequence of which they are rendered capable of a certain degree of contraction, 

 according to the affection of the internal surface by the application of stimuli, as well as of subsequent 

 dilatation according to the subsidence of the action of the stimulus ; thus admitting and propelling the sap 

 by alternate dilatation and contraction. In order to give elucidation to the subject, let the tube be supposed 

 to consist of an indefinite number of hollow cylinders united one to another, and let the sap be supposed 

 to enter the first cylinder by capillary attraction, or by any other adequate means; then the first cylinder 

 being excited by the stimulus of the sap, begins gradually to contract, and to propel the contained fluid 

 into the cylinder immediately above it. But the cylinder immedi.itely above it, when acted on in the same 

 manner, i's affected in the same manner ; and thus the fluid is propelled from cylinder to cylinder till it 

 reaches the summit of the plant. So also when the first cylinder has discharged its contents into the 

 second, and is no longer acted upon by the stimulus of the sap, it begins again to be dilated to its original 

 capacity, and prepared for the introsusception of a new portion of fluid. Thus a supply is constantly kept 

 up, and' the sap continues to flow. The above is by far the simplest as well as most satisfactory of all 

 theories accounting for the ascent of the sap. 



1548. Contraction and dilatation. Knight has presented us with a theory which, whatever may be its 

 real value, merits at least our particular notice, as coming from an author who stands deservedly high in 

 the list of phytological writers. This theory rests upon the principle of the contraction and dilatation, not 

 of the sap-vessels themselves, as in the theory of Saussure, but of what Knight denominates the silver 

 grain, assisted perhaps by heat and humidity expanding or condensing the fluids. {Phil. Tra?is , 1801.) 

 Keith considers this theory of" Knight as beset with many difficulties, and the agency of the alleged cause 

 as totally inadequate to the production of the effect to be accomplished. 



1549. Necessity of an equilibrium in the plant. Du Petit Thouars attributes the motion of the sap to an 

 inherent power, with which nature has been pleased to endow vegetables. But the cause of the renewal 

 of its motion in the spring, after remaining in a quiescent state for several months, he ascribes to the 

 necessity of maintaining a perfect equilibrium in the system of a plant. So that, if a consumption of sap 

 is produced at any given point, the necessity of making good the space so occasioned consequently throws 

 all the particles of sap into motion ; and the same effect will continue to operate as long as any 

 consumption of sap takes place. The first cause of this consumption of sap he declares to be the deve- 

 lopement of the buds, and already formed young leaves, by the stimulating action of light and heat, but 

 particularly of the latter. As soon as this developement occurs, an assimilation and absorption of sap is 

 occasioned for the support of the young leaves, a vacancy in the immediate vicinity of the leaves is 

 produced, and a motion immediately takes place. {Londoti Eticyc, art. Bot.) 



1550. Electricity. The most satisfactory hypothesis for the ascent of the sap is that of M. Dutrochet. 

 This philosopher, by careful examination with a microscope, found that the minute conical termination 

 of the radicle was furnished with other projecting bodies, like sponges, which perform the office of the 

 piston of a syringe, and have the power of introducing into their cavity, and through their sides, the 

 water which comes in contact with the exterior surface, and which spongioles oppose, at the same time, 

 the exit of any fluid which they may imbibe. The motions of the sap and juice in plants take place, 

 according to this author, in consequence of the operations of two distinct currents of electricity : the 

 one negative, by which the vessels have the power of absorption, which M. Dutrochet calls endosmose, 

 and by which the vessels become turgid ; and the other positive, by which the vessels exude or secrete, 

 which" power M Dutrochet calls exosmose. {Gardener's Mag., vol. iii. p. 78. ; Dutrochet, Agent Immediat 

 du mouveinent vital, Paris, 8vo, 182b'.) 



1551. Elaboration of the sap. The moisture of the soil is no sooner absorbed into the 

 plant than it begins to undergo a change. This is proved by the experiment of making 

 a bore or incision in the trunk of a tree during the season of bleeding ; the sap that issues 

 from the wound possesses properties very different from the mere moisture of the soil, 

 as is indicated by means of chemical analysis and sometimes also by means of a peculiar 

 taste or flavour, as in the case of the birch tree. Hence the sap has already undergone a 

 certain degree of elaboration ; either in passing through the glands of the cellular tissue, 

 which it reaches through the medium of a lateral communication, or in mingling with 

 the juices contained in the cells, and thus earning oft' a portion of them ; in the same 

 manner, we may suppose, that water, by filtering through a mineral vein, becomes im- 

 pregnated with the mineral through which it passes. But this primary and incipient 

 stage of the process of elaboration must always of necessity remain a mystery to the 

 phytologist, as being wholly eftected in the interior of the plant, and consequently beyond 

 the reach of observation. All he can do, therefore, is to trace out its future progress, 

 and to watch its succeeding changes, in which the rationale of the process of elaboration 

 may be more evident. 



1552. The process of elaboration is chief y operated in the leaf: for the sap no sooner reaches flu leaf, 

 than part of it is immediately carried off by means of perspiration, perceptible or imperceptible ; effecting 

 a change in the proportion of its component parts, and by consequence a change in its properties. 



1553. Hales reared a sun-flower in a pot of eartli till it grew to the height of three feet and a half; he 

 then covered the mouth of the pot with a plate of lead, which he cemented so as to prevent all evaporation 

 from the earth contained in it. In this pk»te he fixed two tubes, the one nine inches in length and of but 

 small diameter, left open to serve as a medium of communication with the external air; the other two 



