534 



SECRETORY AND EXCRETORY SYSTEMS 



B 



processes (particularly in connection with protein synthesis) ; but this 

 substance is poisonous to the protoplasm, and is accordingly rendered 

 innocuous by combination with calcium to form the very insoluble 

 oxalate of that metal. In certain cases, however, calcium oxalate de- 

 posits are redissolved and once more taken into the metabolic cycle ; 

 this process, which has been observed by De Vries, Tschirch, Schimper, 

 and others, usually takes place when there is a deficiency of calcium in 

 the plant. 



There can be little or no doubt that calcium oxalate, especially 

 when deposited in the form of acicular crystals or of raphides, frequently 



performs an ecological function by pro- 

 viding mechanical protection against 

 noxious animals. Stahl has been fore- 

 most in endeavouring to demonstrate 

 this protective role of raphides with 

 the aid of a variety of ingenious ex- 

 periments. Lewin, on the contrary, 

 adduces experimental evidence to show 

 that these structures are harmless to 

 animals, though he admits that they 

 may help to effect the injection of 

 poisonous substances, just like the 

 brittle points of stinging-hairs. 



While it is probable that Stahl has 

 exaggerated the protective value of 

 raphides, there are a number of cases 

 in which the shape of the raphide-sacs, 

 the properties of the mucilaginous 

 sheath investing the raphide-bundle, 

 and the mode of thickening of the cell-wall, clearly indicate 

 that the raphide-sac is specialised as a protective organ. The 

 following instance will serve to emphasise this point. The single- 

 layered plates of parenchyma that make up the ventilating tissue 

 (aerenchyma) of the leaf of Pistia Stratiotes, are traversed at frequent 

 intervals by spindle-shaped raphide-sacs which project into the air- 

 chambers on either side (Fig. 215). The walls of these sacs are fairly 

 thick, except at the two ends, which are obtuse, and covered by an 

 exceedingly thin membrane. When a sac is mechanically injured (the 

 mere access of water is not sufficient), the raphides are ejected with 

 considerable force generally one at a time owing to the swelling of 

 the mucilaginous envelope ; the thin apical membrane is pierced by the 

 needles as they pass out and soon disappears. In such a case as this, 

 the point of exit of the raphides is evidently predetermined by the 



Fig. 215. 



A. Raphide-sac from the leaf of Pistia 

 Stratiotes, in the intact condition ; the blunt 

 tips of the cell are very thin-walled. B. 'Yrp 

 of an injured sac, more highly magnified ; 

 some of the acicular crystals have been 

 forcibly ejaculated. 



