CALCIUM OXALATE 533 



solitary crystals and sphaerraphides are, in many instances, encased in 

 a closely-fitting occasionally suberised or lignified cellulose membrane, 

 which is connected with the cell-wall at one or more points (leaf of 

 Citrus). Sometimes a crystal appeal's to be suspended in the cell- 

 cavity by cellulose trabeculae {e.g. the so-called " ltossanoffian crystals," 

 found in the pith of Kerria japonica, in Ricinus communis, in the 

 petioles of the Aroideae, etc.)- Each individual member of a raphide- 

 bundle is likewise enclosed in a delicate sheath, which is at first of 

 a protoplasmic nature, but which subsequently becomes converted into 

 a very resistant substance of unknown chemical composition. The 

 whole raphide-bundle, in its turn, is embedded in a mass of mucilaginous 

 material, which is formed in the interior of a special vacuole ; this sub- 

 stance has the property of swelling rapidly in contact with water. 

 Zacharias and Uothert have shown that the walls of crystal- sacs are 

 not uncommonly suberised, an inner suberin-lamella being laid down 

 upon an outer unsuberised layer. 



The arrangement of crystal-cells is determined by a variety of 

 factors. Like other excretory organs, they tend to be associated with 

 the parenchymatous conducting tissues, in the primary and secondary 

 cortex, in the pith, in the " nerve parenchyma " and in the vicinity of 

 leptome-strands and vascular bundles generally. On the other hand, they 

 are also frequently located in the immediate neighbourhood of mechanical 

 cells and strands ; the advantage of this arrangement, no doubt, depends 

 upon the fact that no interchange of material takes place between fibres 

 and the adjoining conducting parenchyma, so that the presence even of 

 large numbers of crystal-cells at the boundary between these two 

 tissues is entirely unobjectionable. Specialised crystal -sacs rarely 

 occur in the epidermis, although solitary crystals or spherical aggregates 

 of calcium oxalate are not uncommon in epidermal cells. Mobius has 

 noted the occurrence of multicellular scale-hairs on the pistil of Cocos 

 nucifera, which contain raphide-bundles in their large marginal cells. 

 Crystal-sacs may be isolated or arranged in rows ; the latter condition 

 is exemplified by the raphide-cells of most Commelyxaceae and 

 Amaryllidaceae and of many Liliaceae. In the secondary phloem 

 of many woody plants, the seriation of the crystal-sacs is due to the 

 fact that cambial cells become divided by transverse walls into a number 

 of vertically superimposed segments, each containing a single crystal or 

 spherical aggregate. Such crystal-fibres, as Hartig has termed them, 

 may comprise a small number of cells, or may, on the other hand, be 

 made up of as many as 20 to 30 segments (Fig. 214 b). 



As a general rule the calcium oxalate deposited in crystal-sacs 

 represents an excretory product from the physiological point of view. 

 Oxalic acid is formed in plants as a result of a variety of metabolic 



