422 SUGAR AND OILS. PART n. 



of trees, as the lime tree, are probably intercepted by the 

 dialysing septa of the cells, and exude to the exterior 

 through the pores of the skin. The sweet juice found 

 in the nectaries of flowers is formed in other parts of the 

 plant, and rarely flows to the flower before it is full 

 blown ; the quantity is at its maximum during the emis- 

 sion of the pollen, and ceases when the fruit is formed. 

 In dioecious plants and that singular and beautiful race 

 the Orchidese, it is evidently intended to attract insects 

 for their aid in fertilization. 



Vegetable oils, resins, and wax, consisting of the same 

 simple elements as the hydrates, form a large class of 

 inflammable organic substances in which hydrogen pre- 

 dominates. Olive oil is a rare instance of a fixed oil 

 being obtained from a fruit ; some laurels have that pro- 

 perty also, but the fixed oils are chiefly found in seeds, 

 as the walnut, hazel nut, and the almond, in which the 

 principle of oil is in its greatest purity. It is particu- 

 larly abundant in hemp seed, and in a great variety of 

 plants the starch in the seed is changed into oil to nou- 

 rish the embryo, till the seed lobes are above the ground, 

 and the true lobes appear. 



Resins, gums, and wax, being colloid substances, are 

 dialysed and ejected from the system either through the 

 fissures in the bark, or by pores in the leaves. The 

 resins exude through the bark from canals that run 

 between the cells of the plant, in solution, and are con- 

 solidated by the oxygen on coming into the air. The 

 herbaceous zone in the bark of the fir and pine family 

 furnish an abundant supply of resins and balsams ; the 

 camphor tribe and the Amyrids are rich in them, as 

 frankincense, myrrh, balm of Mecca, and the Olibanum, 

 supposed to be the frankincense of scripture. 



Wax is a frequent vegetable production, especially in 

 the torrid zone, where many of the wax-bearing plants 

 supply the natives with light. An exudation through 

 the pores of many plants coats their surfaces with resin 



