CYTOLOGY 35 



cells. On the other hand, such a cell may continue to 

 add to the material laid down in its cell- wall, and may 

 do this to such an extent that the wall attains a great 

 thickness and the cell may become what is called sclerised. 

 Sometimes the cell elongates meanwhile, and a long, 

 thick-walled fibre is formed. By the modifications of 

 the cell- wall also, the much elongated and complex vessels 

 of the vascular tissues are characterised. Several cells 

 fuse together, end to end, for their formation, and the 

 walls are thickened and sculptured in many different 

 ways. When such modifications have taken place the 

 protoplasm and nuclei of the cells die, and no further 

 development is possible. The cells which retain the 

 power to divide and form new tissue, whether it be in 

 the wood-forming cambium, in the stem-growing tip, 

 or in the sexual organs, such cells remain soft-walled 

 and undifferentiated. In all such cases of division and 

 the formation of new cells the prime mover is the nucleus. 

 While it is at rest the structure of the nucleus appears 

 comparatively simple. It is composed of a granular 

 mass with one or two large and more definite bodies 

 within it, the nucleoli, and between it and the cell 

 protoplasm is a fine wall, the nuclear membrane. But 

 when the impulse to divide has stirred in it its structure 

 changes, and the granular substance kaleidoscopically 

 becomes a long thread coiled many times on itself. In 

 the meantime the nucleoli disappear, then the thread 

 breaks up into short segments of equal length termed 

 chromosomes. By this time faint striations are seen 

 radiating from two poles in the nucleus, and the little rod- 

 like lengths of the original thread arrange themselves on 

 the equator of the striations. They gradually split and 

 move apart from one another, equal numbers going 

 to each pole. When they have reached this a line of 



