420 



FARMERS' REGISTER 



[No. 7 



For the Farmers' Register. 



ESSAY ON VEGETABLE PHYSIOLOGY. 



By George D. Armstrong, Prof, of Nat. Philosophy 

 and Chemistry, in Washington College, Va. 



[Continued from page 334.] 



Chapter XV. 



MEANS WHICH PLANTS POSSESS OF PERFORM- 

 ING THEIR VITAL FUNCTIONS. Isl. CAPIL- 

 LARY ATTRACTION; 2nd. ENDOSMOSis; 3icl. 

 IMMEDIATE VITAL ACTION. 



Having given, in the preceding parls of this 



could assist as but liltle in our present inquirj-. 

 For this reason I shall confine mysell'to an expiu- 

 nation of theseveral means by vvlucii the organs of 

 plants are now believed to perlbrni their vital 

 functions, and aflerivards, to determine, so lai- as 

 the circumstances of the case permit, the extent 

 to which tiiese means aie severally employed. 



The phenomena of vegetable Jile are, at the 

 present day, generally ascribed, in part to capil- 

 lary attraction, in part to endosmosis, and in part 

 to motions in the organs of plants, (paused imme- 

 diately by the vital principle. We will examine 

 these in the order in which they have been men- 

 tioned. 



1st. Capillary attraction. This is that species 



of attraction manifested by the rising ol" liquids in 



essay, some account of the general structure of I small tubes, or between parallel plates placed at 



plants, and of the nature of those elementary or 

 gans by means of which they are enabled to per- 

 fb; ni their vital functions, to absorb nourishment 

 fron) the earth and from the atmosphere, and ap- 

 propriate it to the production, increase and repair 

 of their different parts, we will now attempt to 

 detpnnine the means by which these organs are 

 enabled to perform their vital functions. Plants 

 may be consid.ered as a set of machines, by which 

 the common elements of nature are worked up 

 imo such a form as to be fit lor the sustenance of 

 animal life. We have already examined the 

 structure of this machine ; we will now direct our 

 attention to the way in which it operates. "In 

 this department of the science, the difficulties 

 vvhicfi the philosopher has to overcome are of a 

 very different character from those which may 

 have embarrassed him in merely determining the 

 organization of a plant. In the latter case, good 

 microscopes, manual dexterity in preparing the 

 parts for examinuiion, and sufficient patience for 

 his task, are sui-e to brln;.' the observer to conclu- 



very small distances from each other. The most 

 important of those laws which govern this species 

 of attraction, are, 1st. it is only in tubes of les.s 

 than one-lenth of an inch in diameter, that this 

 attraction is manifested to any considerable de- 

 gree ; and it is in those tubes which are not larger 

 than a hair, that it is most conspicuously mani- 

 fested ; 2nd, the height to which water will rise 

 in capillary tubes of different diameters, varies 

 inversely as those diameters; 3rd, that a tube j^^ 

 of an inch in diameter will raise water 5. 3 inches. 

 And since the height varies inversely as the dia- 

 meter, the product of the diameter into the height is 

 a constant quantity, viz. the .053 of an inch. Froiu 

 these data it is easy to determine the height to 

 which a lube of any given diameter, will raise 

 water; or knowing the height to which water ?s 

 raised by anv tube, to determine the diameter of 

 that lube. These are laws which have been de- 

 duced from the results of experiments carefully 

 made and frequently repeated, and we must take 

 them as our guides in attempting to determine the 



sions, the general truth of which is often suscopiible j part which capillary attraction act?, in the econo- 

 of exact demonstration; but when we come to con- [ my of vegetable life. 



sider the causes of vital phenomena, and the man- 

 ner in which they are broiight about, we Iiave 

 obstacles of quite another kind to overcome. 

 There is not a function of vegetable life which is 

 not preformed as it were beiiind a screen ; the 

 p;irts which are the prime movers in every ope- 

 ration, are so minute as to escape our view, until 

 they have been killed lor microscopic examination — 

 fixed to the soil, destitute of passions and sensa- 

 tions, the visible expres.^ions of which might lead 

 U3 to ih.e discovery of their invisible causes — 

 having the whole of its organic mechanism con- 

 cealed beneath a skin inert and opaque— we are 

 compelled to trust tor all our notions of the manner 

 in which a plant performs its vital functions, to 

 induciions from data about which, in many cases, 

 there must always, liom the naturt- of things, be 

 some kind of uncertainty. In such circumstances, 

 can we wonder that (rreat d,veisity of opinion has 

 existed among physioloijisis respecting' many of 

 the phenomena of vegeuihie lile ; or ihnt multi- 

 tudes of erroneous theorits liave obtained belief 

 almost without question." 



It would be incouisistent with the character of 

 tlie present es.^ay, to notice the different theories 

 which have been successively advanced and aban- 

 doned; such an account would properly form a part 

 of the history of vegetable physiology, and not of 

 science itself; and whatever interest'i't miirht pos- 

 sess, or whatever instruction it might afford, it 



The rise of the sap of plants has long been as- 

 cribed principally to capillary attraction; and its 

 descent to the attraction of gravitation. There 

 are several verj^ weighty objections to this way of 

 accounting for the circulation of the sap. The 

 first and greatest objection is, that capillary attrac- 

 tion is entirely inadeijuate to produce the effect 

 ascribed to it. From a consideration of the third 

 general law of this attraction, as ijiven above, it 

 will at once be seen that the diameter of a tube 

 being known, the lieii^ht to which water will rise 

 in it can be ascertained by dividing .033 by that di- 

 ameter expressed m decimals of an inch. The 

 most common diameter ol" the ducts of plants, 

 the only vessels which are continuous tubes, 

 is about .002 of an inch. Dividmir .053 by 

 this, the result is 2G.5 inches. The smallest o'f 

 these vessels is about .001 of an inch in diameter. 

 Such tubes would (;| course raise water 53 inches, 

 or to just ilouhlo the height of the former. But 

 the sap of plants, instead of bring raised only 26 

 or at most 53 inches, is, in the case olsome of the 

 large trees of tropical countries, raised to a height 

 of 5000 inches. Another objection, and an unan- 

 swerable one, to this as a general Iheorv, is that a 

 whole natural flimily of phuits. those termed cellu- 

 lar plants, are entirely destitute of tubular vessels; 

 and yet their circulation goes on with as great, and 

 in many instances, even greater rapidiiy'tiian that 

 of other plants. There is no plant which grows 



