i64 CHEMICAL COMPOSITION OF PLANTS 



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portion of whose oil has been extracted from them by crushing 

 and other means. 



Ex. 91.— Cut thin sections of the seeds of the almond, rape, Brazil-nut 

 and linseed. Mount in water and examine with a high power : note the 

 round bright oil-drops in the cells, and in the water round the section. 



3. Volatile or essential oils. — To these compounds are due 

 the aroma or odour of various plants, such as roses, mint, hops, 

 and lavender. 



Many essential oils are composed of carbon and hydrogen 



only, while others contain oxygen 

 in addition to these elements. 

 They frequently occur in the 

 form of drops in the cytoplasm 

 of the cells, and are sometimes 

 accumulated and deposited in 

 special parts of glandular hairs 

 and other receptacles. 



4. Organic acids. — The com- 

 monest examples of these com- 

 pounds found in the cells of 

 green plants are oxalic, malic, 

 citric, and tartaric acids. They 

 are met with either in the free 

 state, or combined with various 

 organic or mineral bases to 

 form acid and neutral salts. 



The commonest acid in plants is oxalic acid, which occurs, 

 free, or more commonly combined with calcium or potassium, 

 in the parenchymatous tissue of leaves, stems and roots ; to the 

 acid potassium salt, is due the sour taste of the leaves of Sorre) 

 {Rumex acetosd) and Wood-sorrel (Oxalis acetosella). 



Crystals of calcium oxalate are very common in the tissues 

 of a great variety of plants ; they are formed in vacuoles within 

 the cytoplasm, and occur in the form of (i) single crystals {a, 



Fig. 8o.' — a Single large crystals of 

 calcium oxalate in cells of the parenchyma 

 of a red clover leaf; b crystal-aggregates 

 from a rhubarb leaf; c raphides from a 

 leaf of a fuchsia. 



