160 The Anatomy of Plants BOOKII 



day strange to read how completely the new theory had 

 captured the adherence of the leading minds ; no less a 

 thinker than Sachs wrote ' the foundations of von Mohl's 

 theory of growth in thickness were shaken in 1858 by 

 Naegeli's observations, and we may say that on the whole 

 it has been for ever superseded '.* 



The lines of research adopted after 1860 were certainly 

 varied, but among them five features stand out very 

 prominently. The first of these centred about the cell. 

 The study of its protoplasm was pursued with great eager- 

 ness, and its constitution, both chemical and physical, gave 

 rise to considerable speculation. The discovery of much 

 more extensive structural differentiation than had been 

 thought possible followed the great improvement of histo- 

 logical methods that were derived to a considerable extent 

 from the laboratories of animal physiologists. The interest 

 which in the earlier time had centred round the cell wall 

 was now transferred to the protoplasm. The nucleus, dis- 

 covered long before by Robert Brown, had remained almost 

 an unknown quantity, but gave up its secrets so far as 

 structure was concerned to the investigators of the period, 

 among whom Strasburger was pre-eminent. The extension 

 of the idea of intercommunication of cell-substance and 

 so of unity of the protoplasmic basis of the plant, based 

 on the discovery or rather proof of the perforations of the 

 sieve tubes shown by Naegeli in 1861, and more clearly 

 by Hanstein in 1864, led to the discoveries of Gardiner 

 and of Russow that protoplasmic threads permeate all cell 

 walls. The mysterious changes of karyokinesis, or mitosis, 

 which mark the division of the nucleus, when brought to 

 light proved but the commencement of researches leading 

 to the establishment of results of the greatest importance 

 concerned with questions of heredity and descent. In this 

 way the study of the cell initiated a movement which 

 1 History of Botany, Eng. ed., p. 304. 



