SECRETION 461 



Some of these processes of catabolism or, as it is some- 

 times called, destruoiive metabolism; are directly applied to the 

 production of substances which are. of great use to the plant. 

 The formation in this way of materials which are of value to 

 the organism is generally called secretion. In other cases 

 bodies are produced which are of no value to the plant and 

 which are as far as possible withdrawn from the spheres of 

 vital activity. Comparatively few of them are ever thrown off 

 from the plant, but they are generally deposited in such regions 

 as leaves which are about to be shed, or the bark of trees, 

 which is a collection mainly of dead matter, or sometimes in 

 special cells, or in ceU-waUs, or elsewhere. These bodies 

 reaUy correspond to excreta, and the processes of their formation 

 are called' processes of excretion. Many others occur whose 

 meaning is still obscure, and these constitute what are often 

 called the bye-products of metabolism. 



Of the processes of secretion the most prominent perhaps 

 is that of the formation of cell-walls. We have seen that in 

 the division of a cell in any of the higher plants the nucleus 

 divides by a series of complicated movements, and forms two 

 daughter nuclei, which are at first connected together by a 

 structure called the sjiindle. Along the fibrils of this spindle 

 minute particles or granules, originating in the protoplasm, 

 pass to form a plate of extreme tenuity across the cell midway 

 between the two new nuclei. This plate soon undergoes a 

 transformation into the ordinary cellulose of the plant. The 

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

 secreted by it, the granules of which it is at first composed 

 being the result of decomposition 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. Fresh 

 granules can be seen to be deposited hy the latter iipon the 

 surface of the original cell-wall, ^\-hich are soon transformed into 

 the first thickening layer of cellulose. In all cases, there- 

 fore, the formation of cellulose can be traced to the self-decom- 

 position of the protoplasm. 



A very similar phenomenon is observable in the formation 

 of starch grains. In naost cases this is brought about by the 

 activity of a plastid, either a chloroplast or a leucoplast. We 

 have seen that these structures may be regarded as specially 

 differentiated protoplasmic bodies, and their mode of behaviour 

 has been described. Building themselves up at the expense of 

 sugar and probably nitrogenous residues remaining in the 



