222 



THE CABINET OF NATURAL HISTORY. 



We have now seen that the growth of plants, and their 

 increase in size, depend upon a peculiar internal movement, 

 acting between the leaves and the roots. But in what way 

 does it operate? This is a problem which has exercised the 

 ingenuity of all students of vegetable physiology, who have 

 contrived theories innumerable to explain the phenomenon 

 which is called the circulation of the sap. 



The great and almost impenetrable obscurity in which 

 this subject is unavoidably involved, has occasioned much 

 diversity of opinion among phytologists. Grew states two 

 hypothesis, which he seems to have entertained at different 

 periods, though it is not quite certain to which of them he 

 finally gave the preference. In one of them he attributes 

 the ascent of the sap to its volatile and magnetic nature, 

 aided by the agency of fermentation ; but this hypothesis 

 is by much too fanciful to bear the test of serious investiga- 

 tion. In the other he attributes the entrance and first stage 

 of the sap's ascent to the agency of capillary attraction, and 

 accounts for its progress as follows: the portion of the tube 

 that is now swelled with sap, being surrounded with the 

 vesiculse of the parenchyma, swelled also with sap, which 

 they have taken up by suction or filtration, is consequently 

 so compressed, that the sap therein is forced upwards a 

 second stage, and so on till it reaches the summit of the 

 plants. But, if the vesiculae of the parenchyma receive 

 their moisture only by suction or fikration, it is plain that 

 there is a stage of ascent beyond which they cannot be thus 

 moistened, and cannot, consequently, act any longer upon 

 the longitudinal tubes. The supposed cause, therefore, is 

 inadequate to the production of the effect. 



Malpighi was of opinion that the sap ascends by means 

 of the contraction and dilatation of tlic air contained in the 

 air vessels. This supposition is perhaps somewhat more 

 plausible than either of Grew's; but, in order to render the 

 cause efficient, it was necessary that the tubes should be 

 furnished with valves, which were accordingly supposed; 

 but of which the existence has been totally disproved by 

 succeeding phytologists. If the stem or branch of a plant 

 is cut transversely, in the bleeding season, it will bleed a 

 little from above as well as from below: and if the stem of 

 any species of spurge is cut in two, a milky juice will exude 

 from both sections in almost any season of the year. Also 

 if a plant is inverted, the stem will become a root, and the 

 root a stem and branches, the sap ascending equ.dly well 

 in a contrary direction through the same vessels; as may 

 readily be proved by planting a willow twig in an inverted 

 position. But these facts are totally incompatible with the 

 existence of valves; and the opinion of Malpighi is conse- 

 quently proved to be groundless. 



The next hypothesis is that of M. De la Hire, who seems 

 to have attempted to account for the phenomenon by com- 



bining together the theories of Grew and Malpighi. Be- 

 lieving that the absorption of the sap was occasioned by the 

 spongy parenchyma, which envelopes the longitudinal tubes, 

 he tried to illustrate the subject by means of the experiment 

 of making water to ascend in coarse paper, which it did 

 readily to the height of six inches, and by particular man- 

 agement even to the height of eighteen inches. But, in 

 order to complete the theory, valves were also found to be 

 necessary, and were accordingly summoned to its aid. The 

 sap which was thus absorbed by the root, was supposed to 

 ascend through the woody fibre, by the force of suction, to 

 a certain height ; that is, till it got above the first set of 

 valves, which prevented its return backwards; when it was 

 again supposed to be attracted as far before, till it got to the 

 second set of valves, and so on till it got to the top of the 

 plant. 



This theory was afterwards adopted by Borelli, who en- 

 deavoured to render it more perfect, by bringing to its aid 

 the influence of the condensation and rarefaction of the air 

 and juices of the plant, as a cause of the sap's ascent. And 

 on this principle he endeavoured also to account for the 

 greater force of vegetation in the spring and autumn; be- 

 cause the changes of the atmosphere are then the most fre- 

 quent under a moderate temperature; while in the summer 

 and winter the changes of the atmosphere are but few, and 

 the air and juices either too much rarefied, or too much con- 

 densed, so that the movement of the sap is thus at least 

 prejudicially retarded, if not perhaps wholly suspended. 

 But as this theory, with all its additional modifications, is 

 still but a combination of the theories of Grew and Mal- 

 pighi, it cannot be regarded as afibrding a satisfactory solu- 

 tion of the phenomenon of the sap's ascent. 



With this impression upon his mind, and with the best 

 qualifications for the undertaking, Du Ilamel directed his 

 efforts to the solution of the difficulty, by endeavouring to 

 account for the phenomenon from the agency of heat, and 

 chiefly on the following grounds: because the sap begins to 

 flow more copiously as the warmth of spring returns; be- 

 cause the sap is sometimes found to flow on the south side 

 of a tree before it flows on the north side ; that is, on the 

 side exposed to the influence of the sun's heat sooner than 

 on the side deprived of it ; because plants may be made to 

 vegetate even in winter, by means of forcing them in a hot- 

 house; and because plants raised in a hot-house produce their 

 fruit earlier than such as vegetate in the open air. 



On this intricate but important subject, Linnseus appears 

 to have embraced the opinion of Du Ilamel, or an opinion 

 very nearly allied to it, but does not seem to have strength- 

 ened 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 efl'ect. 



