August 1, 1890.] 



KNO^VLEDGE 



191 



plant life ? It is certainly not the sap. The sap consists 

 of water sncked up by the roots and the products of assimi- 

 lation drawn from the leaves. The sap is entirely liquid, 

 and contains no solid bodies in suspension. It does not 

 circulate through the plant in tubes ; for the substance of 

 a plant is built up of closed cells and vessels. The trans- 

 ference of the sap from cell to cell can only take place by 

 what is called diffusion, that is, by penetrating through 

 the partitions between the cells ; and, moreover, it is pro- 

 bable that the greater part of the water which ascends 

 from the roots to the leaves finds its passage not by this 

 slow diffusion but by permeating continuously the sub- 

 stance of the walls which enclose the cells. It is clear, 

 therefore, that the sap cannot convey solid particles, and 

 conseqiiently the juice ^v'hich exudes fi-om the wounded 

 stem of a nettle cannot be the sap, because, as already 

 stated, it contains minute granules in suspension. 



This juice is known to botanists as the Latex. Unlike 

 the sap, it is contained in continuous tubes or veins, which 

 ramify in innumerable quantities throughout the entire 

 substance of the plant. If it were possible by any means 

 to destroy all the other tissues of such a plant as a large 

 KiijihiirhiK (for the nettle family is by no means the only 

 family in which these latex tubes are found), then, as 

 Sachs has jDointed out, " the entire form of the plant 

 would still be preserved as a mass of very fine threads of 

 various thickness, representing the ramifications of the 

 original latex-cells ; just as the injected vascular system of 

 a vertebrate animal after the removal of all other tissue 

 allows the whole organisation of the body to be recog- 

 nised." From the fact that the latex-cells run in such 

 large numbers through every portion of the tissues it is 

 evident that their contents must play a very important 

 part in the physiology of the plant. The exact nature of 

 that part has not been completely made out, but this much 

 may be said, that it is analogous in some respects to the 

 part played by blood-vessels and veins in animals. In 

 other words, the latex-tubes contain substances of two 

 kinds, those which in combination with the liquids and 

 gases absorbed by the plant are utilised in its growth, 

 and those which are not so utilised but remain in the 

 latex-tubes as excretions or secretions. 



It would be incorrect to suppose, however, from the 

 analogy of the blood-vessels in an animal, that there is 

 anything in the nature of a circulation of the latex in the 

 latex-tubes. When an animal is wounded the blood 

 nnmediately begins to How, in consequence of the pump- 

 ing action of the heart. The fact that when a latex- 

 bearing plant is wounded the latex immediately exudes, 

 might seem at first sight to be evidence of a circulation of 

 the latex. If the bore of an extremely line glass tube be 

 filled with liquid, and the tube be then broken in the 

 middle, there will be no discharge of the liquid from 

 either of the broken ends. Assuming, therefore, that 

 the latex does not circulate we must look for some special 

 reason to explain the fact that it Hows so readily from 

 the smallest incision. 



The cause we seek is to be found in the phenomenon 

 known as " turgescence," and the manner of its action is 

 explained by a simple experiment given in Sachs' admi- 

 rable work on the physiology of plants, if we cut a fresh 

 and nourishing leaf-stalk from a rhubarb plant, we have 

 a fairly rigid and elastic staff which does not bend by its 

 own weight when we hold it hori/oiitally by one end. If 

 a strip of the skin be peeled neatly off from one end to the 

 other, and we then try to lit the piece of skin back into 

 into its place, we find that it is too short. We may infer, 

 tlierefore, that before it was peeled off" it was in a state of 

 tension, and that as soon as it was removed it contracted. 



If instead of removing merely a strip we peel off all the 

 skin, we find not only that the skin becomes shorter, but 

 that the remainder of the stem becomes considerably 

 longer. If the stem was originally a foot long, it may now 

 be as much as thirteen inches. The skin is limp and 

 flexible, and so also, in a less degree, is the soft interior of 

 the stem ; the soft interior has still a certain amount 

 of stiffness, for the substance of the cell-walls and vessels 

 contribute in some measure to the rigidity of the stalk, 

 but neither the skin nor the stripped interior can lie held 

 horizontally without bending. In fact, the stiffness of the 

 iminjured stalk is mainly due to the mutual tension and 

 pressure between the skin and the mass of soft tissue 

 enclosed in it. It is evident, therefore, that the substance 

 of a plant is under strong hydi'ostatic pressure, to which 

 much of its rigidity and elasticity is attributable. A 

 single cell is compared by Sachs to " a thin-walled 

 caoutchouc balloon which, when empty, is a limp, wrinkled 

 sac, but which may be converted into a firm, elastic 

 sphere by being strongly inflated with air." " If we sup- 

 pose," he says, " some hundreds of thousands of small 

 caoutchouc balloons thus inflated with air, and all con- 

 tamed together in an extensible caoutchouc vesicle, the 

 latter with its contents would form a rigid bar. like 

 the stem of a plant. If we suppose the small caoutchouc 

 balloons not inflated with air but fiOed tense with water, 

 the same effect results ; and it is somewhat in this 

 manner that we have to imagine the rigidity of a stem 

 produced by the turgescence of the cells." From this it 

 is sufficiently clear that the expulsion of the latex from a 

 cut surface is caused by the high pressure in the substance 

 of the plant, and we have thus a complete solution of one 

 of the many problems suggested by the latex-tubes. 



ILcttfvs. 



Rcprintid /mill the " Times " of otii Juhi 18110. 



The Gatkway of Lincoln's Ixn. — The following letter 

 has been forwarded to the Benchers of Lincoln's Inn, on 

 behalf of the Society of Antiquaries, by the president of 

 the societv : — " Societv of Antiquaries of London. Bur- 

 lington H'ouse, Piccadilly, W.. .July 2, 1800. To the 

 Benchers of the Honourable Society of Lincoln's Inn. 

 My Lords and Gentlemen, — In December, 188.5, at a 

 meeting of the Society of Antiquaries of London, I was 

 authorised by a unanimous vote of the society to sign a 

 memorial praying you to countermand the threatened de- 

 struction of the gatehouse court of Lincoln's Inn with the 

 chapel, hall, and the interesting chambers known as 

 No. 24, where once lived Secretary Thurlow. Since that 

 time the buildings have been preserved, but it now appears 

 that they are again in danger of destruction. From an 

 independent survey of competent architects, I have reason 

 to believe that the dilapidated condition of the buildings 

 has been nmch exaggerated, and that by a comparatively 

 small expenditure on judicious repairs the buildings may 

 be placed in such a condition that they will last for many 

 generations to come. 1 therefore again venture to raldress 

 you in the name of this society, and to intercede on their 

 behalf for tlie preservation of these interesting memorials 

 of a past age, the destruction of which would be as deeply 

 regretted as it is apparently needless. I have tlie honour 

 to be, my Lords and (ientlemen, your obedient servant, 

 .loHN Evans. President of the Society of Antiquaries of 

 London." 



