CALCIUM CARBONATE. SILICA. 537 



probably the body of a cystolith always contains some silica] in the 

 Moeaceae and Urticaceae the stalk is strongly silicified. 



There are a certain number of less specialised structures which are 

 closely related to typical cystoliths. In the leaf of Ficus carica, for 

 example, isolated groups of cells in the upper epidermis have greatly 

 thickened outer walls, from which knob-shaped processes, impregnated 

 with calcium carbonate, project into the cell-cavity (Fig. 217 c). The 

 cystolith-like structures observed by Penzig in certain Cucukbitaceae 

 are of a similar nature. 



It is probable that cystoliths are generally bodies of an excretory 

 nature. For unknown reasons, plants provided with these structures 

 seem to require a large supply of lime ; special reservoirs are therefore 

 needed for the reception of the quantities of calcium carbonate that 

 become superfluous in the course of metabolism. In special circum- 

 stances, however, the lime deposited in cystoliths may be redissolved 

 and utilised afresh. Thus the author has noted that some of the 

 cystoliths of Ficus carica become completely decalcified when the leaves 

 are emptied of their contents in autumn ; the stalks of these cystoliths 

 remain unaffected, but the bodies finally come to consist of nothing but 

 a shrivelled brown cellulose skeleton (Fig. 217 b). Considerable 

 numbers of such decalcified cystoliths may also be observed in old 

 leaves of Ficus elastica, if the plant has been grown in a small pot for 

 some time, a condition of affairs which is likely to result in a shortage 

 of calcium. In such cases the cystoliths clearly undergo a change of 

 function ; excretory structures become transformed into repositories of 

 reserve-materials, and the temporary surplus of lime is reintroduced 

 into the metabolic cycle. 



6. Deposits of silica. 265 



Silicification of cell- walls is a phenomenon of widespread occur- 

 rence in the vegetable kingdom. For the present it will only be 

 necessary to consider those cases in which special cells usually 

 arranged in approximately spherical groups have definite portions 

 of their walls thickened and silicified. Generally it is the walls which 

 face one another that become silicified in such a group of cells. 

 The compact " silica-corpuscles " (Kiesel-Korper) that arise in this 

 way, generally appear as characteristic translucent patches when the 

 leaf or other organ in which (they occur is examined with a hand- 

 lens. Siliceous deposits of this nature are found in the leaves of many 

 species of Aristolochia. They are generally composed of a small 

 number of epidermal and hypodermal cells ; less frequently they are 

 deeply embedded in the mesophyll. In Loranthus europaeus spherical 

 silica-corpuscles occur close to the tip of the leaf, and also along 



