66 OBSERVATIONS ON THE GROWTH IN DIAMETER OF 



our experiments was furnished in the paper presented at the 

 Evening Meeting : it will suffice to remark that most distinct 

 and vertically isolated formations of cellular tissue were fur- 

 nished by the exposed cells of the " Couche generatrice,''' laid 

 bare by decortication. With respect to the mode of origin 

 of these cellular masses, I have been able plainly to confirm that 

 they are not mere hernia-like prolongations or emissions from 

 the medullary rays,* but result from the division and multipli- 

 cation of the exposed cells, whether of the rays or prosenchym, 

 indifferently. (PI. II., fig. 1.) True, desiccation and ultimate 

 death of the cells of the exposed tissue might prevent this process 

 in the outermost of these, yet, in such cases, a " Couche gene^ 

 ratrice''^ may establish itself where sufficiently protected from 

 evaporation by overlying cells, provided the cells of this subja- 

 cent layer have not their walls too far thickened, nor their 

 vitality too nearly lost.t 



With regard to the scraping of the denuded alburnum, adopted 

 by Trecul in some of his earlier experiments, in order to remove, 

 I presume, tissues, to which a doubt might apply as to their in- 

 dependent origin, I may say, it does appear totally unnecessary. 

 Although, in like manner, our first decortications were so treated, 

 yet by this means, so far as I can perceive, no scientific certainty 

 is attained : we remove an outer layer merely to lay bare a sub- 

 jacent tissue deposited, or rather formed, under precisely similar 

 conditions, and not only so, but to a great extent do we interfere 

 with the successful issue of the experiment. I reported a case 

 (on the Sycamore) in which no means were employed to remove 

 any tissues remaining after the cutting out of a ring of bark, 

 further than wiping, with a cloth, the denuded surface. In this 

 instance, the subsequent formations were much more uniform 

 than in previous cases in which the alburnum was deliberately 

 scraped, although the precaution was fully adopted to intercept 

 the vertical continuity of newly-forming tissues with those both 

 above and below, by sufficiently deep, annular notches. 



* See Gaudichaud, Amiales, 3 Ser. , vol. xviii. 



I" It would be interesting' to investigate how far, and when, in the phanerogamia, the 

 process of deformation, of solution of the secondary laj'ers of old cells, and period of 

 actual death of the Nitrogenous Cell contents (Endoplasts of Huxley) can take place. 



