SECRETION AND EXCRETION OF MINERALS 



207 



pour forth more acid, and an enzyme similar to animal pepsin, 

 by means of which the insect is digested. 



Secretion and Excretion of Minerals. — In some plants 

 single cells, and strands or layers of cells forming a more or 

 less extensive tissue, are devoted to the secretion and excretion 

 of calcium oxalate crystals — excretion in the sense that while 

 the crystals remain within the body of the plant and are con- 

 tained within cells they have been set apart by themselves where, 

 as a rule, their isolation continues throughout the life of the 

 plant. The calcium oxalate crystals occur singly or in groups 

 in a single cell, and either as simple or compound crystals, as 

 shown in Fig. 116. The forms and association of the crystals 

 may be influenced by the strength of the solution in the cell-sap; 

 but evidently the protoplast has a very important influence, for 

 in certain cells and tissues only one kind of crystal may occur, 

 as in the case of the acicular 

 crystals in Tradescantia, 

 Pistia, Arisaema, etc. 



A secretion of relatively 

 rare occurrence is that of 

 calcium carbonate in the 

 cystolith cells of some Mora- 

 cese, Urticacese, and Acantha- 

 ceae, occurring commonly but 

 not solely in the leaves. 

 Following the development of 



a cystolith it is found that the outer wall of an epidermal cell, 

 for instance, grows down into the cell cavity, swells out at the 

 end and there becomes warty at the surface (Fig. 117). 



When a cystoHth is treated with hydrochloric acid it quickly 

 diminishes in size with the evolution of bubbles of COj, show- 

 ing that calcium carbonate is being decomposed. At the com- 

 pletion of the reaction a skeleton of cellulose remains. The 

 cystolith is therefore a cellulose outgrowth of the wall infil- 

 trated and encrusted with calcium carbonate. In nature the 

 calcium carbonate of the cystolith comes and goes, and its secre- 



FiG. 117. — Cystoliths from the leaf of 

 Ficus carica. A, complete cystolith; B, 

 cystolith from which the calcium carbonate 

 has been removed for use in other parts of the 

 plant. B is from a leaf that had fallen off in 

 autumn. (After Haberlandt.) 



