CALCIUM OXALATE 



535 



structure of the cell-wall. The tapering of the sac at both ends prevents 

 all the raphides from being ejected simultaneously ; owing to the suc- 

 cessive ejaculation of the raphides, an animal aggressor may be 

 wounded in a number of different places by a 

 single raphide-sac. 



The protective function of acicular calcium- 

 oxalate crystals is also very clearly illustrated 

 by the so-called stiuging-hairs of certain 

 Euphorbiageae (Dalechampia, Tragia, etc.), 

 which were first described by Crhger, and have 

 more recently been investigated by Ritters- 

 hausen and by Knoll. The latter author 

 describes the structure of a stinging- hair of 

 Dalechampia Roezliana as follows : The axis 

 of the hair is occupied by a " central cell " or 

 stinging-cell proper (Fig. 216 e) which is sub- 

 epidermal in origin; this cell is "15 to "17 mm. 

 in height, and is surrounded for about three- 

 quarters of its length by a sheath of three or 

 four peripheral cells, which represent greatly 

 elongated epidermal elements (Fig. 216 it). 

 The free distal end of the stinging cell is 

 shaped like a slender cone, and ends in a sharp 

 point. It contains a sphaerraphide of calcium 

 oxalate ; one of the component crystals of this 

 aggregate bears a long needle-shaped process 

 which extends to the extreme tip of the 

 stinging cell, where it lies close against the 

 cell- wall. The whole sphaerraphide is enclosed 

 in a cellular sheath which is connected to the 

 cell-wall by a number of trabeculae, and is, 

 therefore, held very securely in position. 



The stinging-cells of Dalechampia agree 

 with those of Urtica in containing; a large 

 amount of protein material dissolved in their 

 cell-sap ; it is probable, therefore, that the 

 chemical composition of the poison is similar in the two cases. As 

 a matter of fact, the minute hairs of Dalechampia are incapable of 

 wounding the human skin sufficiently to cause a stinging sensation. 

 In Tragia, on the other hand, the hairs are much larger and produce 

 quite an appreciable irritation. It is only the acicular crystal that 

 penetrates the skin ; the cell-wall slips backwards over the needle and 

 is thrown into folds. The needle prepares the way for the entrance 



Fu;. 216. 



Stinging hair from a bract of 

 Dalechampia Roezliana ; c, central 

 or stinging-cull ; r, peripheral 



cells. 



