STARCH-GRAINS 41 



is due to the conversion of the soluble carbohydrates (e.g. sugars), 

 which are produced more rapidly than they can be removed, 

 into insoluble starch. In darkness, when carbon dioxide as- 

 similation ceases, the accumulated starch is changed back into 

 sugar and transferred to other parts of the plant. 



In contradistinction to this transitory starch, that which 

 accumulates in storage-organs usually takes the form of rather 

 large grains which originate within colourless plastids, known as 

 leucoplasts. The latter occur in those cells which are not exposed 

 to light, and differ from chloroplasts only in the absence of 

 chlorophyll, which is generally not produced in darkness. Leu- 

 coplasts, however, readily change to chloroplasts. When a 

 Potato-tuber, for Instance, is exposed to light, it turns green 

 through the formation of chlorophyll within the leucoplasts. 

 In a few cases large starch-grains are actually produced within 

 green plastids (e.g. in a small greenhouse plant known as Pelli- 

 onia), and these provide particularly appropriate material for 

 studying the mode of formatiori of the grains. 



Thus, in a transverse section of the stem of Pellionia the 

 outermost cells (Fig. 20, a) are seen to contain chloroplasts (ch.), 

 in some of which there is a bright shining dot, the transitory 

 starch-grain (s). The larger starch-grains, seen nearer the centre 

 in various stages of development, may be supposed to have 

 originated likewise, as small bodies within the chloroplasts there 

 situated (Fig. 20, b). But in the mature condition these grains 

 have enlarged to such an extent that the enveloping chloroplasts 

 appear merely as green caps to one side of them (Fig. 20, c). 

 On staining with dilute iodine both small and large grains take 

 on the blue colouration typical of starch. 



The large starch-grains that can be scraped out of a Potato 

 are more suitable for the study of details of structure. Examined 

 in a drop of water, each shows a number of asymmetrical layers 

 arranged concentrically around the darker hilum, or point of 

 origin (Fig. 20, d). This stratification indicates the manner of 

 growth of the starch-grain, the successive layers being distin- 

 guished, presumably, by differences either in chemical or physical 

 structure. In the grains of the Potato (Fig. 20, d) and Pellionia 

 (Fig. 20, c) the point of origin is towards one end (excentric type). 

 On the other hand, the hilum, in those from the cotyledons of 



