1884.] of the cell-wall and middle lamella. 91 



probability that gums may be regarded as altered mucilages, i.e. 

 as mucilages which have undergone some change in their properties, 

 both in consequence of free exposure to the atmosphere, and of 

 forming certain definite chemical combinations such as occur in 

 gum arabic, which may be regarded as consisting of the arabates 

 of calcium and potassium. Moreover, speaking generally, muci- 

 lages are connected with living, and gums with dead cells. One 

 cannot however, I think, attempt to separate sharply the one 

 class of bodies from the other. Between the typical gum and the 

 typical mucilage, most distinct differences may exist, but there are, 

 connecting the two extremes, many transition forms, which com- 

 pletely and insensibly bridge over the space and prevent us from 

 placing with any degree of definiteness, a strict line of demarkation 

 between the two. In any case, the bodies in question are to be 

 regarded as mainly the result of a degeneration of the cell-wall. 

 In gum-tragaganth the remains of the latter may be detected 

 which moreover gives a blue with iodine and chlor. zinc, iod., 

 but Cherry gum, on the other hand, displays no structure, and with 

 the same reagent, simply stains yellow. With the mucilages which 

 turn blue with iodine we shall deal later on. 



In most cases we have certain microchemical tests, which afford 

 a means of distinguishing the various modifications of cell-wall 

 from one another, and since I may subsequently have occasion to 

 refer to them it would be well that I should give them here. 



Cellulose. Payen 1 , as I have already mentioned, first demonstrated 

 that walls containing cellulose gave a blue colour with iodine and 

 sulphuric acid, and subsequently it was shown that the same reaction 

 took place with iodine and zinc chloride (chlor. zinc, iod.) 2 soluble in 

 sulphuric acid, ammoniacal oxide of copper, or Schweitzer's reagent 3 

 (copper, ammonio-sulphate), it is insoluble in Schultze's mixture 

 (potassic chlorate and nitric acid), and indeed in oxidizing agents in 

 general. Lastly, as I have pointed out 4 , it is especially stained by 

 dilute alcoholic solutions of methylene blue 5 . 



Lignin. In contradistinction to. cellulose, this substance is soluble 

 in oxidizing agents, such as Schultze's mixture, nitric acid, chlorine 

 water, potassic permanganate or chromic acid. Such reagents convert 

 it into resinous acid bodies which are soluble in alkalies. In sulphuric 



1 Payen, loc. cit. 



2 With Eussow's mixture of iodine and chlor. zinc. iod. cellulose walls do not 

 turn blue but simply remain brown. See Sitzber. der Dorpat. Naturf. Gesell. 

 September, 1883. 



3 Schweitzer, Vierteljahrsschrift. Naturf. Ges. Ziirich, Bd. n. 1857. 



4 Gardiner, Phil. Trans. Part in. 1883. 



5 With regard to the manipulation of this dye, it is important, that a dilute solu- 

 tion be made in 50% alcohol; that the staining be quickly accomplished, and that 

 the section be well washed in water before mounting in Glycerine. This treatment 

 also applies to Hofmann's blue. 



