RECENT ADVANCES IN VEGETABLE CYTOLOGY. 33 



of senescence and death. Some cells, indeed, are not really 

 useful to the plant of which they form a part, until they are 

 dead, i.e., till the wall of the cell alone remains, whilst from 

 its cavity the protoplasm has disappeared. 



The researches of Zacharias and of Rosen, which have 

 recently been published, were directed especially to the 

 behaviour of nuclei in the apical regions of plants, and 

 their results in the main are confirmatory of each other, 

 though the two observers were interested in rather different 

 aspects of the same problem. The nuclei of all actively 

 dividing cells are markedly cyanophil, and this character is 

 especially noticeable just below the active generative cells. 

 At first sight it may seem remarkable that in a fern root 

 the nucleus of the large apical cell is less cyanophil than 

 are the nuclei of the dividing segment cells which have 

 been cut off from it. But the anomaly is only apparent, 

 for though all the cells in the root owe their origin ulti- 

 mately to the division of the apical cell, it must not be 

 forgotten that the nuclear divisions in the segments which 

 are cut off from it are far more frequent. The segments 

 divide up into a very large number of cells before they 

 finally form permanent tissue cells, and therefore it is not 

 surprising to find that the nucleus of the apical cell, which 

 is the ancestor of them all, contains less nuclein than the 

 more actively dividing descendants. But there are several 

 other significant observations which go to show that in cells 

 which are in a state capable of further division, this faculty is 

 correlated with the presence of nuclein in their nuclei. Rosen 

 found in the roots of the bean and other flowering plants 

 that after the tissues were beginning to show differentiation, 

 the zone of cells forming the pericycle 1 retained, in their 

 nuclei, the characters of embryonic cells, that is to say, 

 that, whereas the nuclei of the rest were losing their cyano- 

 phil character and were becoming erythrophil, the pericyclic 

 •nuclei retained their nuclein contents. . Now the lateral 

 roots arise in this pericyclic layer, and they do so by the 

 differentiation in it of new growing points. Hence these 



1 A zone of parenchymatous cells sheathing the more central wood and 

 bast parts of the vascular strand. 



3 



