176 HISTORY OF BOTANY 



ravelling the mysterious changes that occurred in the 

 nucleus during cell-division, changes which were spoken 

 of as karyokinesis or mitosis. Here again we owe the 

 foundation of our knowledge to the labours of Flemming, 

 Van Beneden, and Strasburger. Reduction division was 

 first discovered by Van Beneden in the origin of the sexual 

 cells of the worm Ascaris, and later by Overton in a 

 Cycad, while, as I have already told you, Strasburger 

 in 1894 formulated the generalisation that while the 

 number of chromosomes into which the reticulum sub- 

 divided was constant for each plant type, that number 

 was halved in the gametophyte stage, to be reassumed 

 on the fusion of the gametes and maintained throughout 

 the sporophyte, until, in the mother spore cell, reduction 

 division introduced the gametophyte number once 

 more. 



Another subject that was hotly debated during the 

 later years of the nineteenth century was the structure 

 and mode of growth of the cell wall. Naegeli, in his 

 classic work on the starch grain, published in 1860, had 

 put forward the view that organised bodies consisted 

 of " micellae " or invisible crystalline particles, separated 

 from each other by equally invisible shells of water, and 

 that the density of the body as a whole was determined 

 by the thickness of these aqueous shells. Growth in 

 thickness, according to Naegeli, depended on the inter- 

 polation of new micellae between those already present, 

 by a process which he termed " intussusception." Later 

 investigators, such as Strasburger, held on the other 

 hand that the particles were not crystalline, and that 

 they were bound together chemically and not by molecular 

 attraction, as was suggested by Naegeli, while growth 

 in thickness was effected by apposition of new layers, 

 not by intercalation of new particles. 



In 1896 Wiesner advanced yet another theory of the 

 structure of the cell wall. He suggested that the wall 

 was living, being composed of " dermatosomes," as he 



