5*74 APPENDIX. 



the thickening; therefore the layers of thickening must be deposited 

 on the outside. This objection again is refuted by H. v. Mohl, by the 

 help of most accurate measurements and acute reasoning. The third 

 point refers to chemical relations. They are as follows: The entire 

 wall of the young cell re-acts purely as cellulose, for, treated with 

 iodine and sulphuric acid, it becomes blue throughout its entire thick- 

 ness. The older cells exhibit various layers. The most external 

 consist of a matter quite insoluble in sulphuric acid. This membrane 

 is therefore deposited externally upon the original cellulose layer, 

 and closes up the original pores at the outside. The remainder of 

 the layers acquire a colour less blue and more green and yellow in 

 proportion as they lie more externally ; wherefrom Mulder deduces 

 either a disappearance of the cellulose and replacement by new sub- 

 stance, or a deposition of new layers always on the outside of the pre- 

 ceding. Harting, on the contrary, finds in this a proof that the ori- 

 ginally pure cellulose becomes subsequently saturated with an incrusting 

 (proteinous) substance, which accumulates especially in the outer parts. 

 In opposition to these, Hugo von Mohl demonstrates that, in the first 

 place, the results deduced from chemical relations are not conclusive ; 

 and, secondly, that all membranes in the entire plant, all so-called 

 intercellular substance, and the secreted layer of the epidermis, have 

 cellulose for their element, and are only brought to re-act differently 

 to iodine and sulphuric acid by a gradual and varying degree of satura- 

 tion by a foreign matter which penetrates them ; that this inter-pene- 

 trating substance may be removed from all the parts forming the external 

 coverings of plants, e. g. the secreted layers of the epidermis, cork and 

 bark, by action of caustic potash, or from all the internal greatly 

 thickened elements of the plant, e. g. pith, wood, and liber-cells, by 

 boiling in nitric acid ; a single exception occurring in a very delicate 

 lamella on the secreted layer of the epidermis, which remains of a yellow 

 colour under all circumstances, and therefore Mohl wishes the term 

 cuticula to apply exclusively to this lamella. 



In conclusion, I will only observe, that from my own researches I 

 must accede to these results of Mohl's in every respect. 



Page 92., add before 40. 



c. At the period of complete maturation occur in the cells of succulent 

 fruits, the grape, gooseberry, many kinds of Solanum, &c., numerous 

 more or less minute spherical vesicles of extreme delicacy, the walls of 

 which consist of a slightly granular protoplasma, the contents of a 

 watery, and often coloured, juice. So far as I could see, they originate 

 at once of full size, as vesicles or bubbles of the primordial utricle, upon 

 which they are at first flatly applied. Subsequently they separate by 

 constriction. Hartig*, who mixes them up with many other things, calls 

 them metacardial cells. Karsten f confounds them with the ferment-cells. 

 Nao-eli \ enumerates them in part under his abnormal cell-formation. 

 I regard them as altogether dependant forms, incapable of further 

 development. 



* Das Leben der Pflanzenzelle : Berlin, 1844. 

 i Creation (Die Urzeuguny), Botanische Zeitung, 1848, p. 457. 

 Zeitschrift fur wiss. Bot., vols. iii. and iv. (1846), p. 23, et seq. 



