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 CompositcB. 



no. In maiiy 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. Resins 

 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 

 19* 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 ( Oinchona). 



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- 184. InuIinc crystals from the roots of the Thistle. 



