THE CELL-NUCLEUS. 



87 



a watery protcinaccous substance, differs apparently not very essentially from 

 the surrounding protoplasm. Within this is present, however, a second sub- 

 stance in the form of granules or filaments, which, according to certain state- 

 ments, is distinguished by containing phosphorus ^ and appears in a more 

 definite form especially when, at the commencement of cell-division, the cell- 

 nucleus itself prepares to divide. These two substances may perhaps be 

 distinguished most advantageously by the terms nucleoplasm and nuclein. The 

 latter is especially remarkable in that with colouring media, and particularly 

 with Haematoxylin, it stains more quickly and deeply than the nucleoplasm; 

 whence this reagent is also employed to make the nuclei evident, and indeed 

 generally in very large numbers, in cells where no nuclei at all were detected 

 formerly. In the larger fully grown cells 

 of the higher plants, the nucleus appears 

 generally as an inactive mass ; in cells poor 

 in protoplasm usually lying on the wall. 

 Its prominent significance is clear, on 

 the other hand, in two cases. First, in 

 the growing point (Fig. 72), wehere the 

 nuclei almost entirely fill up the space 

 of the otherwise small cells, so that the 

 mass of a growing point consists to a 

 very great extent of nuclear substance. 

 Yet more striking, however, appears the 

 nucleus as an essential element of the cell 

 during cell-formation itself: there results 

 at this time a more definite separation 

 of its two constituents, the nucleoplasm 

 and the nuclein, to which I shall return 

 later. 



The third essential constituent of a 

 vegetable cell, the cell-membrane or cell- fig. 8s.-/'to-,> «7;,,/»,«: structure of ti,e brown scieren- 



11 r 11 ,• 1 .1 chyma in the stem (X ^iTO). ^ a fresh thin transverse section; 



wall, forms, as already mentioned, the ex- ^ the lonRltudinal wall between two cells (fresh), a twisted pit- 



, 1 i-Ji J C t.\, 11 TV '^^■nsX at the lower end; C transverse section in concentrated 



ternal solid boundary of the cell. In its sulphuric acid ;/? longitudinal secti,™ of tl,e wan in sulphunc 



, . ., • i. r r ^c\A; « the middle lamella of the wall ; * second shell ; irthird, 



primitive state it consists Ot a peculiar innermost shell of the wan ;/ pore canals ; nmnen of the cell. 



chemical compound, cellulose, which is 



composed of Carbon, Hydrogen, and Oxygen. This substance is remarkable for 

 its great resistance to the most various chemical solvent reagents ; and its extra- 

 ordinary solidity and elasticity are of especial importance for the plant. The 

 sharp outHne and impressive form of the parts of plants, and their great solidity, 

 though containing enormous quantities of water, depend essentially upon this 

 property of cellulose. Nevertheless, only the very thin cell-membranes in the 

 young parts of plants, and perhaps in older parenchyma, consist of true cellulose, 

 which is moreover always mixed with water and incombustible mineral substances. 

 With increasing age, and according to the physiological work which the 



Zacharias, Über die ehcmisehe Beschaffenheit des Zellkerns, Bot. Zeitg. 18S1, p. 170. 



