METABOLISM 277 



fully considered in a subsequent chapter ; it will suffice to 

 say here that in all ordinary growing points this division 

 of a protoplast into two is followed immediately by the 

 formation of a new supporting membrane between them. 



The division of the cell is preceded by the division of 

 its nucleus, which is attended by a series of complicated 

 movements of particular constituents of its substance. The 

 two daughter-nuclei are situated at some little distance from 

 each other and are connected by a number of delicate fila- 

 ments which are gathered to a point at each end and 

 spread out in the centre, forming what is called the 

 nuclear spindle. This generally stretches completely 

 across the long diameter of the cell. 



During these introductory changes the hyaline proto- 

 plasm becomes more granular, and the granules, technically 

 spoken of as microsomata, are attracted to the spindle 

 fibres. They pass along these fibrils from both regions of 

 the cell and form a plate of extreme tenuity across it, 

 midway between the two new nuclei. This plate soon 

 undergoes a transformation, the granules disappearing and 

 the membrane becoming translucent, and so forming the 

 ordinary substance of the cell-membrane, generally, though 

 perhaps not strictly accurately, known as cellulose. The 

 cell-wall is thus seen to be formed from the protoplasm, or 

 to be secreted by it, the granules or microsomata of which 

 it is at first composed being the result of decompositions 

 set up in the living substance. 



When cell-walls are growing in thickness or in surface 

 a similar decomposition of the protoplasm can be observed. 

 The microsomata or granules are formed in the proto- 

 plasm and are gradually deposited, often in oblique rows, 

 upon the original membrane. They are subsequently 

 changed in appearance and become the first thickening 

 layer of cellulose. The occurrence of the rows of granules 

 frequently leads to the striated appearance which can be 

 noticed on the walls of many fibres, particularly those of 

 the bast of the fibro-vascular bundles. 



