CELL-CONTENTS. STARCH. 



341 



cause of its errhine properties when powdered and used as 

 snuff. It is in the underlying or true epidermis, that the 

 silica occurs. This membrane is permeable by fluids, not 

 by means of pores, but by endosmotic force. 



" The most generally-distributed and conspicuous of the 

 cell-contents is Starch; at 

 the same time it is one of 

 ^reat value and interest, per- 

 forming a similar office in 

 the economy of plants as that 

 of fat in animals. It occurs 

 in all plants at some period of 

 their existence, and is the 

 chief and great mark of dis- 

 tinction between the vege- 

 table and animal kingdoms. 

 Its presence is detected by 

 testing with a solution of 

 iodine, which changes it to 

 a characteristic blue or violet 

 colour. 1 Being insoluble in 



cold water, it can be readily Fig. 137. section of a cane : with ceii- 

 washed away and separated 

 from other matters contained 

 in the cellular parts of full-grown plants. It is often 

 found in small granular masses in the interior of cells, 

 shown in fig. 183 from the garden-rhubarb. Starch-grains 

 are variable in size; the tous-les-mois, fig. 188, No. 5, are 

 very large ; in the potato, No. 14, they are smaller ; and 

 in rice, No. 6, they are very small indeed. Nearly all pre- 

 sent the appearance of concentric irregular circles ; and most 

 of the granules have a circular spot, termed the hilum, 

 around which a large number of curved lines arrange them- 

 selves: better seen under polarised light. Plate YIII.No. 167. 

 Leeuwenhoek, to whom we are indebted for the earliest 

 notice of starch-granules, enters with considerable minute- 

 ness into a description of those of several plants such as 

 wheat, barley, rye, oats, peas, beans, kidney-beans, buck- 

 wheat, maize, and rice ; and veiy carefully describes 

 experiments made by him in order to investigate the 

 structure of starch-granules. Dr. Keissek regards the 



(1) This is not a test for starch when combined with albuminous matters. 



