XXXIV 



These contemporaneous papers brought into notice those phenomena 

 which, under the comprehensive term " plasmolysis," have come to 

 be of such importance in the explanation of the mechanism of plant- 

 movement. The " Starkekorner " is a paper which most strikingly 

 illustrates in how remarkable a degree Nageli combined untiring in- 

 dustry with scientific insight. Taken simply as a descriptive ac- 

 count of the starch-grains, of the variety of their form in different 

 plants, of their structure, and of their chemical composition, it is the 

 most exhaustive that has yet appeared. The preparation of it, which 

 began in 1850, involved, as Nageli tells us in the preface, the micro- 

 scopic examination of the roots and rhizomes of about 800 species of 

 plants, and of the seeds of about 1700 species. But the main interest 

 of this paper lies, not so much in the facts, as in the theories con- 

 cerning the mode of growth and the molecular structure of organised 

 bodies, for which the facts afforded the necessary basis : and further, 

 in the recognition of the importance of the appearance and disappear- 

 ance of starch-grains as an indication of the metabolic activity of the 

 plant. 



In 1846 (' Zeitsch. f. wiss. Bot.,' Hefte 3 and 4, p. 117) Nageli had 

 expressed the opinion that a starch-grain is a vesicle the wall of 

 which becomes thickened, like that of a cell, by the deposition of 

 successive internal layers. This view, which he had formed, he says, 

 whilst " in Irrthiimern der Schule befangen," he now replaces by the 

 well-known theory of "growth by intussusception." This theory, 

 which Nageli extended also to cell- walls, was generally accepted for 

 many years, until the publication of Strasburger's researches (' Ban 

 und Bildung der Zellhaute,' 1882) when a reaction took place in 

 favour of the theory of growth by apposition. At the present time, 

 however, Strasburger admits (' Histologische Beitrage,' Heft 2) 

 that the growth of cell- walls may be effected by the infiltration of 

 material, a process which, as he says, may be termed " intussusception," 

 bnt is not altogether the same as that conceived and named by 

 Nageli, which was inseparably associated with his theory of the in- 

 timate structure of organised bodies, generally known as the "micellar 

 theory." According to this theory, organised structures, such as 

 cell-walls and starch-grains, consist of solid ultimate particles which 

 Nageli at first regarded as being the chemical molecules, but subse- 

 quently ('Das Mikroskop,' 1877, p. 354), as groups of molecules 

 termed " micellae," each of which is, under ordinary circumstances, 

 surrounded by a layer of water. The particles with their watery 

 envelopes are, he argued, held together by the mutual attraction of 

 the solid particles, the attraction of each particle for its watery 

 envelope, and the cohesion of the ultimate chemical molecules of 

 which each particle consists. By acute reasoning based on the study 

 of the physical properties of organised structures, more especially 



