362 LECTURE XXII. 



Euphorbiacese particularly starch-grains) are as a rule cojitained in the latex; 

 ■Knowing how economical the plant is with these — its constructive materials — it will 

 scarcely be assumed that these substances are contained in the laticiferous vessels for 

 any other purpose than to be conveyed to the growing organs. Of course it is not 

 to be forgotten that the laticiferous vessels contain at the same time considerable 

 quantities of secretions — caoutchouc, resins, ethereal oils, alkaloids, etc.; that is, 

 substances not employed in growth but produced as bye-productsv The presence of 

 these substances in the laticiferous vessels by no means proves that the constructive 

 materials likewise present are useless, however, any more than the presence of 

 carbon dioxide and various products of the decomposition ofthe- tissues in the vtink 

 of animals warrants us in regarding the useful constituents of their blood as useless. 

 With this view, which I put forth in 1865; a series of experimental researches by 

 Faivre ' on Ficus elastka, Morus kitba, and - Tragopogon agree. " Nevertheless, here 

 again a rich field still lies open for experimental and microscopical investigation, and 

 its fruiifulness is perhaps enhanced by the fact that very active peptonising ferments 

 have already been discovered in the latex of some plants, e. g. Carica Papaya and 

 Ficus Carica. 



The pressure of the tissues, as evinced by the outflow of fluids from wounds, is 

 observable, however, not only in the case of sieve-tubes and laticiferous vessels : even 

 the succulent turgid parenchyma itself exhibits an exactly similar phenomenon. Any 

 transverse section made With a sharp knife through a succulent stetn, or petiole, shows 

 that far more sap exudes from the cut surface of the parenchyma than could have 

 been contained in the few cells incidentally opened by the section ; the greajter part 

 of this exuding sap is evidently preyed out from parenchyma cells some distance 

 removed from the section. Here, however, the fact is not so simple as in the case 

 of the sieve-tubes, since it depends upon the exudation of sap frotn closed living 

 parenchyma cells, where the expressed fluid must filter, not only through the cell- 

 walls, but, what is more, through their protoplasmic linings, the pressure evidently 

 being afforded by the strong turgescence of the parenchyma cells themselves. 

 However profitable it might be to enter more ih detail into this remarkable phe- 

 nomenon and its consequences, the mere mention of them must suffice here. 



We have to seek the most general cause of the movements of the materials in 

 plants, however, in diosmotic pfocesses. These consist in the attraction between 

 water and soluble substances, which in the case before us undergoes important modifi- 

 cations from the fact that the processes in questitsn must take place through cellulose 

 walls which are clothed with living protoplasm. The most essential facts on this 

 subject have already been mentioned (Lecture XH). Since space does not allow of 

 our entering specially upon the mechanics of the diosmotic processes coming into 

 consideration here, I must content myself with a few general remarks ". 



Every process of diffusion or of diosmosis between a living cell and its environ- 

 ment must, sooner or later, if undisturbed, pass over into a certain condition of 



1 Cf. Faivre on Latex, 'Ann. des Sc. Nat.' 1866, vol. vi (p. 33), and 1869, vol. X (p. 9?)i 

 'Comptes rendus,' 1879 (p. 369) ; and Pfeffer, ' PJlanzen-phydologie'' (p. 325) ; Hansen, Sihungsber. 

 d.ph. med. Ges., Erlangen, 8 Nov. 1880. 



^ With regard to the theory of diosmosis I refer to the detailed account in VikStth' Pfianzen- 

 fhysiologie,' chap, ii, and Sachs' ' Exferimmtal-physiologie,' 1865 (pp. 157^, &c.) 



