THE CACTUS TRIBE. 223 



sarily be deadly to it. The plant, therefore, takes up from 

 the soil on which it grows, a proportionate quantity of 

 Lime, which combines with the oxalic acid, forming inso- 

 luble crystals, which occur in abundance in all the Cac- 

 tac(B. In some species, as for instance, in the Peruvian 

 and old- man Cactus, the plant contains eighty-five per 

 cent, of oxalate of lime. Oxalic acid might certainly be 

 obtained with profit from these plants in the tropics. 



A third peculiarity is exhibited in the globular forms 

 of Melo cactus and Mamillaria, in the structure of the 

 wood, which differs entirely from that of the common 

 ligneous plants. Common wood, for example that of the 

 Poplar, is composed of long wood-cells, the walls of which 

 are quite simple and uniform, and of cells containing air, 

 the so-called vessels, the walls of which are very thickly 

 beset with little pores. Wholly unlike this, the wood of the 

 Cactus, above mentioned, exhibits only short, spindle-shaped 

 cells, inside which wind most elegant spiral bands, looking 

 like little spiral staircases. 



Lastly, the hairs, spines, &c. situated in the places of 

 leaves, deserve a special mention. Generally speaking, 

 three forms may be distinguished, all three usually 

 occurring together on the same spot. The first are very 

 flexible, simple hairs, which form a little flat, soft cushion ; 

 among these is found a bunch of longish but thin spines. 

 These it is chiefly, which, on account of their peculiar 

 structure, make the careless handling of the Cactus plants 

 so dangerous. These little spines are very thin and brittle, 

 so that they readily break off, and are covered with barbed 

 hooks directed backward from the point. When touched, 

 a whole bunch at once penetrate the skin ; if an attempt 

 is made to draw them out, the separate spines break in 

 the skin, and the fragments pierce in other places ; when 



