24 ORGANOGRAPHY. 



by the action of polarised light, when each granule usually ex- 

 hibits a black cross. Those who adopt this view of 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 gene- 

 ral 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 

 distinguish 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 the starch granules, little was 

 known until recently; but now however, by the researches 

 of Criiger it would appear that they are secreted on the inner 

 surface of cavities or vacuoles formed in the general protoplasm 

 of the cell, in the same way as tlie primordial utricle or super- 

 ficial pellicle of the pjrotoplasm secretes cellulose on its outer 

 surface (page 56). Hence we find a ready explanation of 

 a circumstance already noticed when treating of chlorophyll, 

 namely, the common occurrence of starch granules imbedded in 

 that substance; for chlorophyll, as we have seen, is probably 

 nothing more than portions of protoplasm containing a substance 

 colom-ed green under the action of light, so that starch gra- 

 nules may as readily be formed in cavities of this coloured pro- 

 toplasm as in that of any other. 



Raphides. — This name is now commonly applied to crys- 

 tals of any form found in the cells of plants, although the 

 term raphides (which is the Greek for needles) was originally 

 given to those only which were shaped like a needle {fig. 46). 

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

 in all their organs; generally, however, they are most abun- 

 dant in the stems of herbaceous plants, in the bark of woody 

 plants, and in leaves and roots. Although they are com- 

 monly described as occurring only in the cells, they are 

 sometimes found in the vessels also, which are however, formed 

 of rows of cells, as will be hereafter noticed, and in inter- 

 cellular cavities; the occurrence of crystals, however, in the 

 latter position is jjrobably accidental. In some i)lants they 

 occur in such enormous quantities that they exceed in weight 

 the dried tissue in which they are deposited ; this may be 

 es])ecially observed in some Cuctacea'. ; thus Edwin Quekett 

 found in the stem of the Old-nuin Cactus (Cereus senilis) as 

 much as 80 per cent of crystals in the dried tissue. Professor 

 Bailey also found in a square inch of Locust-bark of the thick- 

 ness of ordinary writing-paper, more than a million and a half 



