The Science of Plant Growing 



examination. The interior at first is entirely filled with protoplasm, in 

 the centre of which is a denser, oval body the nucleus consisting of 

 a granular groundwork of protoplasm, denser at its margin, and having 

 a fibrillar network of granules embedded in it. The nucleus plays a very 

 important part in the division of full-grown cells. As the cell increases 

 in size, cavities make their appearance in the protoplasm, filled with cell 

 sap and air, and this continues till the cavities unite and the protoplasm 

 can only form a lining to the wall, with a few 

 bridles connecting it with the layer of proto- 

 plasm surrounding the nucleus in the centre 

 (fig. 3). Streaming movements of the proto- 

 plasm may often be observed in the living 

 cells of various plants (the direction being indi- 

 cated by arrows in fig. 2). The large granules 

 to be seen embedded in the protoplasm are 



chlorophyll or leaf green. The further history of this cell depends on 

 whether the tissue requires more cells or not for its full development. 

 If it does, then the nucleus elongates into spindle form, the protoplasm 

 forms a mass at each end of the spindle, the two masses being joined 

 by threads. A layer of protoplasm (the cell plate) then extends across 

 the cell from wall to wall, and from this layer a new partition is formed 

 simultaneously and continuously. Thus two cells are formed. The 

 common partition later on splits into two, so that each daughter cell has 



Fig. 3. Isolated Cells (1 and 2) 

 with and (3) without Nuclei 

 highly magnified 



123 



Fig. 4. Changes in the Protoplasm of the Cell Nucleus during its Division 



1, The Nuclear Fibrils distributed through the whole Nucleus. 2, The broken-up Nuclear Fibrils 

 arranged as the Nuclear Plate. 3, The elements of the plate separating from one another. 4, The same 

 elements forming two skeins at the poles of the Spindle. (After Guignard.) Very highly magnified. 



its own complete wall and a half of the original nucleus (fig. 4). If the 

 full-grown cell does not intend to divide, the remainder of the protoplasm 

 is used up in thickening the walls, or is drafted away into younger and 

 growing cells. The empty cell is now dead for all time coming, though 

 it may exist for a thousand years or more, if it forms part of the stem 

 of a giant Sequoia gigantea of California. The cell wall at first consists 

 of cellulose, a substance closely allied to starch and sugar, all three being 

 made up of the chemical elements carbon, hydrogen, and oxygen, in 

 different combinations, and all becoming black when burned. 



