32 ORGANOGEAPHY. 



the structure of the starch granule explain the appearance it 

 commonly presents thus ; the rounded spot or hilum being the 

 nucleus of growth, and the concentric lines representing the 

 boundaries of the successive layers of deposit. 



Starch granules vary very much in the distinctness and general 

 appearance of their concentric lines, in the same way as they 

 differ exceedingly in form and size when obtained from different 

 sources : those, however, which are obtained from the same plant 

 are more or less uniform in appearance, so that we may distin- 

 guish under the microscope the different kinds of starch, and 

 refer them to the particular plants from which they have been 

 derived. 



With regard to the origin of starch granules, it would ap- 

 pear from the researches of Crliger that they are secreted on the 

 inner surface of cavities or vacuoles formed in the general proto- 

 plasm of the cell, in the same way as will be hereafter seen, the 

 primordial utricle or superficial pellicle of the protoplasm secretes 

 cellulose on its outer surface (see Cell-development). Hence 

 we find a ready explanation of a circumstance already noticed 

 when treating of chlorophyll, namely, the common occurrence of 

 starch granules imbedded in it: for chlorophyll, as we have seen, 

 is probably nothing more than granules of protoplasm containing 

 a substance coloured green under the action of light, so that 

 starch granules may as readily be formed in cavities of this co- 

 loured protoplasm as in any other. 



Eaphides. — This name is now commonly applied to crystals 

 of any form found in the cells of plants, although the term ra- 

 phides (which is the Greek for needles) was originally given to 

 those only which were shaped like a needle {fig. 62). Eaphides 

 , may be found more or less in all classes of plants, and in all 

 I their organs ; generally, however, they are most abundant in the 

 i stems of hei'baceous plants, in the bark of woody plants, and in 

 i leaves and roots. In some plants they occur in such enormous 

 I quantities that they exceed in weight the dried tissue in which 

 they are deposited ; this may be especially observed in some 

 Cactacese ; thus Edwin Quekett found in the dried tissue of the 

 stem of the Old-man Cactus ( Ccreus senilis), as much as 80 per 

 cent, of crystals. Professor Bailey also found in a square inch 

 of Locust-bark of the thickness of ordinary writing-paper, more 

 than a million and a half of these crystals. The root of Turkey 

 or Russian rhubarb commonly contains from 35 to 40 per cent., 

 hence when chewed it appears very gritty ; and, as this kind of 

 rhubarb usually contains a larger proportion of raphides than 

 any other, this grittiness has been employed as a means of 

 distinguishing it from them. The raphides are commonly con- 

 tained in cells, in which starch, chlorophyll, and other granular 

 structures are absent, although this is by no means necessarily 

 the case. 



