REJTJVENESCENCT5 IN NATURE. 113 



research, recently so industriously devoted to the history of 

 development, has not yet been able fully to solve. The idea 

 of Schleiden, that the leaf is pushed out, as it were, from the 

 stem, the apex first emerging, and the \owev parts being 

 gradually extricated by development at the base,* is devoid 

 of exact demonstration, vs^hich can only be given by a com- 

 plete history of the development, accurately illustrating 

 the succession of the individual cells, of the apex of the 

 stem, and the rudiments of the leaves arising from it, 

 which certainly exist before they are elevated as papillae. 

 The assertion of Schleiden, that the leaf is formed by a 

 process of cell-formation advancing from the apex to the 

 base, has been contested by Nageli,f who, resting on 

 complete observations on the formation of the leaf in the 

 Floridese, Characese, and Mosses, j sets up the contrary 

 doctrine, that the original growth of the leaf advances by 

 cell-formation at the apex and at the circumference, that 

 is, from the base upwards and outwards, and that only 

 the later growth, connected with final completion of the 

 structure of the leaf by expansion of the cells, begins at 

 the point and circumference, and progresses towards the 

 base. How far these results, derived principally from 

 the Cryptogamic plants, are applicable to the leaf-forma- 

 tion of the Phanerogamia, can scarcely be safely deter- 

 mined, from the deficiency of researches on the latter. 

 That supplementary cell-formation, presenting itself in all 

 or only isolated parts of the first cellular structure, fol- 

 lowing the original cell-formation (doubtless progressing 

 according to accurately defined rules in the higher no 

 less than the lower plants), that supplementary develop- 

 Hient which Nageli calls abnormal or accidental, appears 

 to' be of great importance to the ulterior development of 



* Schleiden, ' Grundziige du Wiss. Botanik,' ii, 113, (2d Ausg.) ' Prin- 

 .oiples of Scientific Botany' (London, 1849), p. 26]. 



f 'Ueber das Wachsthum und den Begriff des Blattes,' Zeitschr. f. wiss, 

 Botanik, 1847, 153; ' Wachsthums-geschiohte der Blatter der Laubmoose,' 

 Ibid, 1845, p. 175. 



% With regard to the two last-named families, I can fully confirm Nageli's 

 observations through my own. 



