530 SECRETORY AND EXCRETORY SYSTEMS 



endodermis, but also occurs in individual cells of the pericycle, 

 which may or may not be in direct contact with endodermal 

 elements. 



The fact that certain poisonous or pungent substances are only 

 produced by enzyme action when the plant is injured, clearly indicates 

 that the bodies in question serve for protection against animal foes. 

 This argument was indeed put forward long ago by Nageli. The usual 

 peripheral location of enzyme-reservoirs, and their frequent association 

 with vascular bundles, also point in the same direction. 



4. Crystal-sacs. 262 



The vast majority of the genuine crystals that are found in 

 vegetable tissues consist of calcium oxalate. Small quantities of this 

 compound occur in the most diverse tissue-elements. But there are 

 also special crystal-sacs or -reservoirs, characterised by the fact that 

 calcium-oxalate crystals always form the bulk of their contents and 

 often fill their cavities completely, or nearly so. 



Calcium oxalate forms crystals which belong either to the mono- 

 clinic (clinorhombic or monosymmetrical) or to the tetragonal (quad- 

 ratic) system. In either case the precise mode of crystallisation varies 

 enormously. We may distinguish between the following principal 

 types. 



(a) Solitary crystals. Solitary crystals of calcium oxalate are 

 developed as quadratoctahedra, if belonging to the tetragonal system, 

 but as hendyohedra when they are monoclinic. Numerous deriva- 

 tive forms, produced by twinning or otherwise, are also known. Such 

 large perfect crystals, which are usually found singly in the cells, 

 occur, for example, in the leaves of Iridaceae and Pontederiaceae, in 

 the phloem of the Pomaceae, Robinia Pseudaeacia, Ulmus campestris, 

 Aesculus Hippocastanum (Fig. 214 b), spp. of Acer, etc. Excellent 

 specimens of twinned crystals are to be seen in the second layer of 

 the testa of Phaseolus vulgaris. 



(b) Crystal dust {Krystall-sand). Crystal dust consists of exceed- 

 ingly numerous minute crystals, the angles and edges of which can 

 scarcely be distinguished, even under the highest magnification ; this 

 form of calcium, oxalate occurs, for instance, in the foliar organs of many 

 Solanaceae and in the primary phloem of the stem in the Cupressineae 

 and in Sambucus, Cinchona, Liriodendron, etc. 



(c) Raphides. Raphides are long acicular crystals, usually aggre- 

 gated in considerable numbers into a tightly-packed bundle or sheaf, in 

 which all the crystals lie parallel to one another and are, as a rule, 

 of approximately equal length. Typical raphides are particularly 

 characteristic of the foliar organs of Monocotyledons ; they 000111- 



