12 ANATOMY AND PHYSIOLOGY OF 



Observ, It is now no longer wortii while to give an hi&torical review 

 of tlie opinions that had })een expres&ed as to the strticture of the cell- 

 wall and of the spofcs, before the appearance of mj es&ay " On the Pores of 

 Vegetable Cellular Tibsne/' in 1828. But it is necesbary to advert to the 

 objections which have recently been advanced by Harting and Mulder 

 against my doctrine of the structm-e of cellS; and of the gradual and 

 successive deposition of the secondary layers from without inwards. 

 f/See Harting " Mlhroclieyyi. Onderzoekmgen^'' &c.j in the " Tidshrift voor 

 naturlijke gescMedemSy" XI. J (translated in the Linndia XIX. Harting : 

 " Letter to If. v. Mohl^'—Bot. Zeitwig, 1847, 337. — Mulder '^Physiological 

 Ghemistrij.'' — Mohl " On the Growth of Gell Membranes,'' — Bot. Zeitung, 

 1846j 337.) I believe I may safely leave unnoticed the objections ad- 

 vanced by Hartig. (^" Beitrage zur Entwickelungsgesch. der PJlanzenP 

 1843; "Das Leben der Fflanzenzelle.''' J 



Mulder and Harting attack my theory on both anatomical and chemi- 

 cal grounds, and seek to demonstrate that the cell-membx^ane increases in 

 thickness in the direction from within outwards by the deposition of 

 layers upon the outside of the original membrane, which process of growth 

 is followed, in some cases, by a deposition in the interior of the cavity of 

 the cell, while in particular instances (in the cells of horny albumen) the 

 membrane itself grows thicker by the interpenetration of foreign matfcer. 

 In the first place, my opponents deny that the thin membranes of the 

 young cell are imperforate, and that only the subsequently internally 

 deposited layers are porous, since they, on the contrary, believe, that they 

 found the membrane of young cells to be perforated like a sieve, while a 

 perfectly closed membrane is deposited subsequently on the outside of 

 these closed cells. It is, of course, not for me to decide who observed most 

 correctly, I or Harfcing ; but I must stand by the facts I have stated, and 

 do not believe that Harting would have been deceived in the manner he 

 has, if, instead of selecting only cells having small pits for his observa- 

 tions, he had extended his researches also to cells with large pits, between 

 which the secondary membranes ^i-ppear in the form of narrow fibres ; and 

 had properly regarded the analogy which exists between the structure of 

 the vascular utricles and cells. Harting finds a second reason for his 

 view of the external growth in his micrometrical measurements of young 

 and of thickened cells (Linncea, 1846, 552), by which he arrived at the 

 conclusion that the cavity of the wood-cells expands during the increase 

 of thickness of a shoot, in exactly the same proportion as the unlignified 

 cells, whence he argued that the thickening of their walls is to be ascribed 

 to a deposition taking place upon the outside of then' primary membrane. 

 On the other hand, I consider that I have demonstrated by my measure- 

 ments (^' Bot. Zeitung'' 1846, 358) that exactly the contrary occurs, and 

 that the thickening of the walls is combined with a narrowing of the 

 cavity of tbe cell. — Mulder and Harting deduce a third counter-evidence 

 from the chemical reaction of the cell-wall (which will be sjjoken of here- 

 after). The membrane of young cells is coloiured blue by the action of 

 iodine and sulphuric acid ; in full-grown cells this very often happens only 

 to the innermost layers, while the intermediate acquire a green or yellow, 

 and the outermost membrane a brown, colour, altogether withstanding the 

 solvent power of sulphuric acid, which is not the case with the interme- 

 diate and inner layers. From this my opponents draw the conclusion 



