RECEPTACLES FOR SECRETIONS. 177 



Crucifera, Ranunculacem ; and Taxus, among- the Conifers otherwise rich in 

 secretions. 



I now pass to a somewhat more detailed description of the most important forms of 

 receptacles for secretions, and shall confine myself here also to the more common cases. 



The commonest of all the receptacles for secretions are the vesicles containing 

 crystals (Lithocysts) in which calcium oxalate is accumulated as refuse matter. 

 More rarely this occurs— ^as in many Solanea, some species of Amaranihus, in 

 the pith of Sambucus nigra, etc. — in the form of exceedingly small crystalline 

 particles, which are deposited in countless, multitudes in individual cells, and fill 

 these entirely. More frequently the oxalate of lime appears in the form of very 

 thin needles, pointed at both ends, which lie parallel to one another and form 

 bundles of so-called raphides, often filling np the entire cavity of elongated vesicles. 

 This is to be observed in many Monocotyledons (Aroidese, Liliacese) ; as well as 

 in some Dicotyledons (the Vine and its allies. Cinnamon, Impattens, etc.). In many 

 Monocotyledons (species of Allium, Irideae, Amaryllideae) and in the great majority 

 of Dicotyledons (particularly abundant and well developed in the bark of Guiacum 

 officinale), the calcium oxalate appears in -the form of single crystals, well formed on 

 all sides, which are contained in the cells as simple individuals; or as twin crystals, 

 or as clusters of crystals. The great variety of forms in which this salt appears 

 is partly explained frj6m the fact that, according to the rapidity of its separation, it 

 crystallises either with two molecules of water of crystallisation in the klino-rhombic 

 system, or with six molecules of water of crystallisation in the quadratic system. The 

 much more frequent klino-rhombic forms in the plant are referred to the ground 

 form of the hendyohedron, and develope as prisms, tables and twins with truncated 

 angles of the most various kinds. The raphides also apparently belong here. The 

 fundamental form of the calcium oxalate crystallising in the quadratic system is the 

 quadrate-octahedron, the main axis of which is sometimes extremely short, so that 

 the crystal assumes the form of a letter cover ; in other cases combinations of the 

 quadratic prism with the corresponding pyramid occur, and so forth \ As a rule 

 the crystal-vesicles contain a slimy substance: if the crystals are very large, the 

 slime is less abundant. 



In some cases, on the other hand (tubers of Orchids), the cell is chiefly filled 

 with slime, and contains only a small group of crystals. Where the crystals of 

 oxalate appear as well-formed individuals or clusters, they often stand in relation with 

 the. cell- wall, and are embedded in cellulose projections, or in the cell-wall itself 

 (pith of Kerria, Ricinus, in the vascular bundles of the petiole of various Aroidea, 

 leaf parenchyma oi Hoya carnosa, leaves oi Citrus, cortex of Salix, Populus, Celtis, 

 Fagus and others). In other cases also a relation of this kind is at least not im- 

 probable. Very small crystals of calcium oxalate — their form no longer distinctly 

 recognisable, but rendered evident by means of polarised light— are also found 

 occasionally deposited in thickened cell-walls. These are particularly common in the 

 bast of Coniferse, and in a few Dicotyledons (leaves of Sedum, Mesembryanthemum). 



1 As regards the crystals of calcium oxalate, the chief work is that of Holzner, ' Flora,' 1864, 

 p. 273, and 1867, p. 499. Further, Rosanoff, 'Bot. Zeitg.' 1865 and 1867. Graf Solms-Laubachj 

 'Bot. Zeitg.' 1871 ; and Pfitrer, 'Flora,' 1872, p. 97, are also important. 



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