CELL-CONTENTS AND FORMS OF CELLS 225 



(Fam. Cochlospermaceae), has been used as a substitute and 

 adulterant of tragacanth. 



VOLATILE OILS AND RESINS.- -These and related products, 

 known as gum-resins and balsams,* are found in a very large 

 number of plants. Like the mucilages, they originate either as 

 a metamorphosis of the cell-wall or as a direct product of the 

 protoplasm. The former is of more frequent occurrence, and the 



B 



FIG. 123. Development of a lysigenous secretory canal in the leaves of Dictamnus 

 albus (Fam. Rutaceae). The development begins partly in the cells of the epidermal layer 

 and partly in the underlying parenchyma (A). The outer cells divide, forming the 

 secretion cells (c), while the inner give rise to the reservoir (B). The innermost cells then 

 multiply by repeated division in all directions, giving rise to a large number of cells containing 

 globules of oil (C). Later the thin walls are absorbed and the oily globules fuse together, 

 forming a single large globule (D). After Rauter. 



The volatile oils are not infrequently associated with other sub- 

 stances of the plant cell in varying proportions, as resin, gums, cinnamic 

 and benzoic acids. Those products which consist chiefly of oil and resin 

 are known as OLEO-RESINS, and include turpentine and copaiba; those 

 consisting chiefly of gum and resin and containing but little volatile oil 

 are known as GUM-RESINS, and include ammoniac, asafoetida, galbanum, 

 and myrrh ; oleo-resins associated with aromatic acids are known as 

 BALSAMS, as balsam of Tolu, balsam of Peru, storax, and benzoin, which 

 latter is usually termed a balsamic resin. 



15 



