86 HISTOLOGY AND PHYSIOLOGY. 



194), which consist of crystalline elements disposed in a 

 radiate manner. Inuline is abundant in the tubers of 

 Elecampane, common Sunflower, roots of Dandelion, and 

 other Compositce. 



no. In many plant-cells, particularly in seeds, fixed 

 oils, as Olive, Castor, Linseed, and Palm Oil, are secreted 

 and exist in the form of drops, mingled with the other cell- 

 contents. The essential oils and resins are generally 

 the products of special cells. Of the former. Oil of Tur- 

 pentine, Oil of Lemons, Oil of Thyme, represent one class ; 

 Camphor, Essence of Cinnamon, etc., a second ; and the 

 essential oils in Mustard, Onions, Asafoet- 

 ida, etc., represent a third class. Resius 

 are generally associated with, and dissolved 

 in, the essential oils. They may be sepa- 

 rated by heat, when they take the form of 

 transparent, or translucent, little bodies. 

 Oxalic acid is generally combined with 

 calcium, in the form of crystals. Other 

 194 vegetable acids are: malic acid, abun- 



dant in many sour fruits. Apples, Cherries, Strawberries, 

 etc. ; tartaric acid occurs in the Grape, unripe berries 

 of Mountain Ash, etc. ; citric acid is abundant in the 

 Lime and Lemon, also associated with malic acid in 

 Gooseberries, Cherries, etc. ; tannic acid, in the bark and 

 leaves of Oak, Elm, Willow, etc. ; quinic acid, nearly 

 related to the last, occurs in Peruvian Bark ( Cinchona). 



III. Calcic oxalate may occur in cell-walls, or within 

 the cell, either in granules, or in the form of crystals. 

 Of the latter, the needle-shaped forms are called raphides 

 (Fig. 195) ; they occur in the cavities of parenchymous 



Fig. 104. Inuline crystals from the roots of the Thistle. 



