264 VEGETABLE PHYSIOLOGY 



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. 



In all cases therefore the formation of cellulose can be 

 traced to the self-decomposition of the protoplasm, though 

 whether the granules are actually cellulose or an inter- 

 mediate substance is still uncertain. 



A very similar phenomenon is observable in the forma- 

 tion of starch grains. In this case, as we have seen, we may 

 either have to deal with the general protoplasm of the cell, 

 or, as is usual in reservoirs and in ordinary leaf parenchyma, 

 with a definite plastid, either a chloroplast or a leucoplast. 

 These structures, however, may be regarded as specially 

 differentiated protoplasmic bodies. We have already 

 discussed their behaviour and the formation of the starch 

 grain by them. Building themselves up at the expense of 

 sugar and probably of various nitrogenous compounds, 

 either brought to them or remaining in their substance, 

 they break down again to a certain extent, splitting 

 off a quantity of starch, which is deposited in the interior 

 of the plastid, sometimes at one point, sometimes at 

 several. As the process goes on, successive laminae or 

 shells of starch are continually deposited round the original 

 grain or granule till the structure of the fully formed starch 

 grain is reached. In this case the process is somewhat 

 clearer than the corresponding one in that of cellulose, as 

 there is little doubt that each shell is composed of starch 

 at the moment of its deposition. 



The formation of starch is in these cases a secretion by 

 the plastid, just as that of cellulose is a secretion by the 

 protoplasm of the cell. The formation of the small starch 

 grains by the general protoplasm of cells in which no 

 plastid is present is of a similar character, though it is not 

 so long continued and the formation of successive laminae 

 does not take place. 



