62 THE ELEMENTARY STRUCTURE OF PLANTS. 



small proportion more of oxygen. (Hydrocyanic or Prussic Acid 

 is one of the special products peculiar to certain plants, and of 

 very different composition, containing a large proportion of nitro- 

 gen.) These vegetable acids do not appear to play any leading 

 part in vegetation. They seldom exist in a free state, but. are 

 combined with the alkaloids, and with the inorganic or earthy al- 

 kalies (Potash, Soda, Lime, and Magnesia) which are introduced 

 into plants from the soil with the water imbibed by the roots. 

 The more soluble salts thus produced are found dissolved in 

 plants ; the more insoluble are frequently deposited in the cells in 

 the form of 



91. Crystals or Rapllides, These exist in more or less abundance 

 in almost every plant, especially in the cells of the bark and leaves, 

 as well as in the wood and pith of herbaceous plants. Far the 

 most common, and the principal kind formed with a vegetable 

 acid, are those of oxalate of lime. In an old stem of the Old-man 

 Cactus (Cereus senilis), the enormous quantity of 80 per cent, of 

 the solid matter left after the water was driven off was found to 

 consist of these crystals. In the thin inner layers of the bark of 

 the Locust, for example, each cell contains a single crystal, as is 

 seen in Fig. 57. And Professor Bailey, who has devoted particu- 

 lar attention to this subject, computed that, in a square inch of a 

 piece of Locust-bark, no thicker than ordinary writing-paper, there 

 are more than a million and a half of these crystals. There is 

 frequently a group of separate crystals in the same cell ; or a con- 

 glomerate cluster, as in Fig. 58. In the leaves of the Fig, and 

 many other Urticaceous plants, a globular crystalline mass is sus- 

 pended in the cell by a kind of stalk. Oxalate of lime crystal- 

 lizes in octahedra (as in Fig. 56, the crystal in the lower right- 

 hand cell), and in right-angled four-sided prisms (as in Fig. 59, 

 60), with variously modified terminations. The crystals are fre- 

 quently acicular, or needle-shaped, either scattered or packed in 

 bundles of from twenty to some hundreds (as in Fig. 53-55). It 

 is to this form that the name of Raphides (which is the Greek 

 word for needles) was originally applied, and to which it properly 

 belongs; although it has been indiscriminately extended to all 

 kinds of crystals which occur in the cells of plants. In the com- 

 mon Arum or Indian Turnip, as well as in the Calla ^Ethiopica and 

 other plants of that family, the crystal-bearing cells (Fig. 54) may 

 readily be detached from the rest of the tissue ; and when mois- 



