VEGETABLE SECRETIONS. 



in cells, but could not produce tlie needle-shaped crystals. He took a portion of rice- 

 paper, and placed it in lime-water under an air-pump, in order to fill the cells with the 

 fluid. The paper was then removed and dried, and the process repeated until the cells 

 were filled. After this the paper was immersed in weak solutions of oxalic and phos- 

 phoric acids, and the crystals appeared at the end of three days (Fig. 91). This, 

 however, is a mere chemical experiment, and has no relation to vegetable tissue, 



Fig. 89. Fig. 90. Fig. 91. 



Fig. 89. Raphides. A mass of crystals from the cuticle of a Cactus. 



Fig. 90. Raphides from the bark of the LIME TREE (Tilia Europva), of considerable breadth and 



prismatic figure. 

 Fig. 91. Crystals of oxalate of lime raphides, produced artificially in the cells of rice-paper. 



except in so far that a detached morsel of vegetable structure was used as the containing 

 vessel. 



Oils and Fats. The most widely distributed of all vegetable secretions, next to 

 that of starch, is essential and fatty oil, of various degrees of consistence ; and, with the 

 exception just referred to, none has so high a value for economic purposes. 



There are probably few, if any, plants from which some portion of oil cannot be 

 obtained by distillation ; but it is more particularly in the hot climates of India, China, 

 New Holland, Africa, South of Europe, and South America, that they attain their 

 highest degree of perfection, and are found in the greatest abundance. The mustard- 

 seed, for example, which is grown in our climate, yields oil only in a non-remunerative 

 degree ; but in the continent of India, with its burning sun, the produce is of great 

 value. So also with the otto or atar of roses an exquisite volatile oil, obtained from 

 the rose-leaf growing in Persia, but scarcely perceptible in our northern climate. This 

 is doubtless due to the chemical influence of the sun's rays, by which all vegetable 

 secretions become highly elaborated. 



The oil is most commonly found in the seeds, as in the linseed and rape-seed, of our 

 climate ; for as the seed is the product of the plant in its most mature condition, it is 

 the most fitted to be a depository of the most mature secretions. It is, however, found 

 to a great extent in the leaves of plants, as the rose and the peppermint, and in the 

 wood of a comparatively few trees for example, the Sassafras and the Sandal- wood. 

 The bark is not an unfrequent depository of oil secretions. 



A recent discovery made by Mr. Young, of Scotland, has demonstrated the wonder- 

 ful length of time during which vegetable oils retain their distinctive characters. He 

 has obtained by distillation, at a low red heat, no less than 20 per cent, in weight of 



