132 



THE PHYSIOLOGY OF PLANTS 



prisms (raphidcs), or flat lozeBge-shaped tablets, or rhomboid 

 crystals, often united in clusters. This oxalate of lime is 

 absent from very few plants (Ferns and Horse-tails) ; the 

 crystals are generally in the cell-sap, but sometimes in the 

 cell-wall substance (Fig. 22). 



In this figure (22) we see two star-shaped cells taken from 

 a septum, which separates the air chambers of the stalk of 

 Musa. In each of the cells, which are separated by large 

 intercellular spaces (i), we see several of these prismatic 

 crystals. Besides these we find that some cells of Musa con- 

 tain the oxalate of lime in 

 the form of raphides (b). 

 While the latter occur most 

 frequently in Monocotyle- 

 dons, Dicotyledons usually 

 have octohedral crystals, or 

 clustered crystals of this 

 substance ; as an example 

 of these, we have the cells 

 (c) taken from the leaf-stalk 

 of a Begonia (after Luerssen), 

 It is more rare to find 

 crystals of calcium carbonate 

 in the tissues of plants ; 

 this generally forms an 

 incrustation on the outer 

 surface (many Algee), or it 

 enters directly into the 

 substance of the cell-wall 

 and renders it resistant. In some natural orders {Urticaceoe, 

 Acanthacece, Cumtrhitacecc), the epidermis and other tissues 

 have club-shaped or rod-shaped processes of cellulose {cystoliths) 

 (Fig. 17, c), which have become covered with carbonate 

 of lime, which, however, does not make its appearance if the 

 plants are grown without lime or without light. 



Only in very rare cases does the lime occur in plants as 

 gypsum (sulphate of lime) or as phosphate of lime. This latter 

 compound sometimes makes its appearance when some tissues 

 which are very rich in protoplasm (potato-peels) begin to 



Fig. 22.— Diffekent Forms of Crystals of 

 Oxalate of Lime. 



