Vegetable Physiology. 



Fit 



substance into two or more, each mass forming its own new cell- 

 membrane (fig. 8, a). 



It becomes requisite here to direct attention to a body almost 

 invariably existing in young cells in connexion with the proto- 

 plasm, and persisting throughout the life of many parenchymatous 

 tissues, namely, the cell-nucleus, to which some physiologists attri- 

 bute the highest importance in the origination of cells, but the real 

 office of which is not yet ascertained. The nucleus (fig. 8) is 



a body usually of a lenticular 

 form, or the shape of an old- 

 fashioned watch, composed, so far 

 as can be determined by tests 

 applied under the microscope, of 

 the same substance as proto- 

 plasm. In full-grown cells where 

 it occurs, it either lies upon the 

 inside of the formative layer {pri- 

 mordial xitricle), or is connected 

 with this by threads of viscid pro- 

 toplasm (c), holding it suspended 

 in the cavity of the cell. The 

 relative size of the cell-nucleus 

 and the full-grown cell will be 

 best understood from the drawings 

 (fig. 8). It is found that where 

 the cell-nuclei exist in cells in 

 course of multiplication, the first 

 step of the process is the division 

 of the nucleus into two (or, as 

 some affirm, the solution of the 

 nucleus and the formation of two 

 new ones). Such a process even 

 forms the forerunner of the division in Spirogyra, described in 

 our former paper, where the drawing (fig. 7, page 79 of vol. xvii.) 

 shows the two nuclei suspended by protoplasmic threads in the 

 central cavity. 



But the relations of the nucleus and its divisions, whatever may 

 be their import, are seen most strikingly in the development of 

 the minute cambial cells of the higher plants. In tiiese the 

 nascent cells are often scarcely larger than the nuclei, which are 

 formed nearly of their full size in the parent-cell before division. 

 This is very well seen in epidermal hairs, where the succes- 

 sive stages of development are presented at one view (fig. 8). 

 In the end cell the nucleus (or a pair of nuclei, if division is 

 about to be repeated) nearly fills the young cell, the formative 

 layer being represented by a thin stratum of viscid protoplasm 



Vei-y young hairs from the surface of buds of 

 AchimeMes : a, the haii', a conical cell filled 

 with protoplasm (retracted from the wall 

 through the action of water), containing two 

 nuclei, which nearly fill it ; (), an older hair, 

 in which the upper part resembles the cell a, 

 while the lower is more developed ; c, a more 

 advanced form, composed of several cells 

 which have expanded considerably, the pro- 

 toplasm which originally filled the cavity 

 being now " honeycombed " by cavities filled 

 with watery cell-sap; the nuclei adhere to 

 the strings of protoplasm which stretch across 

 the cavity of the cell, and are all connected 

 with a thin layer lining the cellulose wall. 



