302 MANUAL OF BOTANY 



occur in such enormous quantities that they exceed in weight 

 the dried tissue in which they are deposited : tliis may be 

 especially observed in some Cactacefe. The raphides are usually 

 contained in cells from which starch, chlorophyll, and other 

 granular structures are absent {fig. 658), although this is by no 

 means necessarily the case. 



The raphides may occur singly in the cells, as in those of 

 the bark of the Locust tree {fig. 656) ; but far more commonly 

 there is a number of crystals in the same cell. In the latter 

 case they are usually either placed side by side, as in the stem 

 of Rumex (fig. 658) ; or in groups radiating from a common 

 point, and so assuming a clustered or conglomerate appear- 

 ance, as in the stem of the common Beet (fig. 657). The former 

 have been termed acicular raphides, and the latter conglomerate 

 raphides or sphceraphides. 



In the common Arum, where raphides are very abundant, 

 and in some other Aracese, the cells which contain the raphides 

 are filled with a mucilaginous sap, so that when they are moist- 

 ened with water endosmosis takes place, by which they are 

 distended and caused ultimately to burst and discharge their 

 crystals from an orifice at each end {fig. 659). 



The crystals thus described consist generally of calcium 

 oxalate, which crystallises in two forms according to the propor- 

 tion of water it contains. "When the crystals contain six equiva- 

 lents of water of crystallisation, they form octahedra (fig. 657), 

 as in the conglomerate raphides or spheeraphides ; when, on the 

 other hand, there are only two equivalents of water of crystallisa- 

 tion, bundles of acicular crystals or true raphides are produced 

 {figs. 658 and 659). In rarer cases the crystals are composed of 

 calcium carbonate. 



The raphides are usually regarded as waste products, or bye- 

 products of the metabolism of the cell. They are frequently 

 surrounded by a delicate peUicle of cellulose, thus being shut off 

 from contact with the protoplasm. 



Of the substances which are met with in solution in the cell- 

 sap, little more need be said at present. The kinds of sugar 

 found are chiefly maltose, grape sugar, and cane sugar. A pecu- 

 liar carbohydrate body, inulin, is found in the tubers or tuberous 

 roots of certain of the Compositse, particularlj' the Dahlia and the 

 Artichoke. This substance has the same percentage composition 

 as starch, and, Uke the latter, is readily convertible into a form 

 of sugar. It can be made to separate out from the cell- sap by 

 keeping the tissue containing it in spirit. A section taken through 



