ELEMENTARY STRUCTUKE. 



of these crystals. The root of Turkey or Kussian rhubarb com- 

 monly contains from 35 to 40 per cent, hence when chewed it 

 appears very gritty; and, as this kind of rhubarb contains coni- 

 monly a larger proportion of raphides than any other, this gritti- 

 ness has been employed as a means of distinguishing it from 

 them. The raphides are commonly contained in cells, in which 

 starch, chlorophyll, and other granular structures are absent, 

 although this is by no means necessarily the case. 



The raphides occur either singly in" the cells in which they 

 are found, as in the inner bark of the Locust-tree {fig. 

 44) ; or far more commonly there are a number of crystals 

 in the same cell, in which case they are usually arranged in 

 one of two ways, that is — either placed side by side as in the 

 stem of Kumex {fig. 46); or in groups radiating from a com- 

 mon point, and forming a clustered or conglomerate appearance. 



Fig. 44. 



Fig. 45, 



Fig. 46. 



'"i i{ 



Fig. 44. Raphides in the cells of the inner bark of the Locust tree. After 



Gray Fig. 4.5. Conglomerate rapliides Fig. 46. Aeicular raphides. 



Two cells contain raphides, and three of them chlorophyll. 



as in the cells of the stem of the common Beet {fig. 45). 

 In the common Arum and some other Arace«, the cells which 

 contain the raphides are filled with a thickened sap, so that 

 when they are moistened with Avater, endosmose takes place, 

 which distends them and causes them ultimately to burst and 

 discharge their crystals from an orifice at each end {fig. 47). 

 Such cells have been called Biforines 



In many plants belonging to the families of the Urticaceae, 

 the Moracere, and Acanthaceae, thei*e may be frequently ob- 

 served situated generally just beneath the surfaces of the leaves, 

 or sometimes more deeply, peculiar crystalline structures, to which 

 the name of Cystolithes has been appMed by Weddell. They 

 constitute the gummi keulen of Meyen. These consist of an 

 enlarged cell containing commonly a globular or clnb-shaped 

 mass of crystals {figs. 48 and 49.) suspended fi-om the top 



