SECT. xrsr. E APHIDES. 425 



crystals of the same substance abound so much in the 

 common rhubarb that the best specimens of the dry me- 

 dicinal root contain as much as thirty-five per cent, of 

 them ; while certain aged plants of the cactus tribe have 

 their tissues so loaded with them as to become quite 

 brittle. The calcareous base in some instances is com- 

 bined with tartaric, citric, or malic acid. The crystals of 

 some raphides are -^th of an inch long, others are not 

 more than the hundredth ; they are brought into view 

 by polarized light. 7 Spherical raphides between the 

 ^JL_th and 4 * O th of an inch in diameter have been 

 discovered scattered profusely through the tissues of the 

 leaves, and those parts of plants which are modifications 

 of the leaves ; they may be seen under the skin of Pelar- 

 goniums and other plants, and it is supposed that few if 

 any orders of plants are without them. 8 



Although azote forms 788 thousandth parts of the 

 atmosphere, none, or at least no appreciable quantity of 

 it, is absorbed by the vegetable world ; that great prin- 

 ciple of nourishment is entirely supplied by ammonia 

 and nitric acid, imbibed by the roots, and decomposed 

 by the chemico-vital power. Here it shows its capri- 

 cious character by combining with other simple ele- 

 ments in the bark, to produce the most precious medi- 

 cines in some plants, and in others the most deadly 

 poisons, while no vegetable substance is perfectly nutri- 

 tious without it. 



The milk sap, when exposed to the air, coagulates 

 into a tenacious viscid solid. The white juice is gene- 

 rally acrid, or narcotic, or both, and for the most part 

 extremely poisonous, though exhibiting strong contrasts 

 even in nearly allied species. In the order Euphorbi- 

 acese or Spurge worts, comprising nearly 1,500 species, 

 a large proportion are hurtful ; but there is a gradation 

 from mere stimulants to the most formidable poisons. 



7 Dr. Carpenter, ' Microscope.' 



8 Annals of Natural History for 1863. 



