536 



SECRETORY AND EXCRETORY SYSTEMS 



of the poison ; it thus plays the part which Lewin attributes to 

 raphides in general. 



5. Cystoliths 



The great majority of Ukticaceae, Moraceae and Acanthaceae, 

 many Combretaceae and Boraginaceae, and certain isolated genera, 

 such as Phlox, Khtgia, etc., develop on some of their cell-walls peculiar 

 calcified thickenings which are termed cystoliths. In the Ukticaceae 

 and Moraceae (excepting Pilea, Matostema and Myriocarpa), these 

 bodies are restricted to the foliar epidermis ; here the outer walls of 

 certain epidermal cells generally distinguished by their larger size 

 are provided with stalked spherical or ovoid processes covered 

 c with projections, which may be 



blunt or more or less sharply 

 pointed. The swollen part of the 

 process may thus be compared to 

 a mulberry suspended from the 

 outer wall by its stalk. The 

 cystoliths often fill the cell-cavities 

 almost completely. These struc- 

 tures are particularly well de- 

 veloped in certain species of 

 Ficus (F. elastica, F. carica, etc., 

 cf. Tig. 217 a). Here the cysto- 

 lith-cell arises directly from a 

 protodermal cell, which remains 

 undivided while the adjacent cells 

 undergo several tangential divi- 

 sions and become transformed into elements of the water-tissue. The 

 cystoliths of the Acanthaceae are usually more or less spindle-shaped, 

 with a laterally attached stalk, which is usually short and weak and 

 often difficult to distinguish. They are not confined to the epidermis 

 of the leaf, but occur also in the parenchymatous tissues of root, stem 

 and leaf. 



It has already been explained that cystoliths are strongly im- 

 pregnated with calcium carbonate. If the mineral substance is dissolved 

 by treatment with acid, an organic basis composed of watery cellulose- 

 material is left behind ; this skeleton always shows concentric stratifi- 

 cation, to which, in the Moraceae, is superadded a system of radial 

 fibrillar striae corresponding to layers that are relatively rich in 

 cellulose. According to Zimmermann, the very similar striae in the 

 spindle-shaped cystoliths of .the Acanthaceae correspond to layers 

 which contain less cellulose than the rest of the cystolith. Most 



Cystoliths of Ficus Carica. A. Cystolith from the 

 abaxial epidermis of the leaf . x280. B. Decalcified 

 cystolith from a fallen leaf (in autumn). C. Group 

 of cells in the adaxial epidermis, with their outer 

 walls thickened, impregnated with calcium car- 

 bonate and furnished with cystolith-like processes. 



