'218 PHYSIOLOGICAL BOTANY. PART II. 



tics. By cultivation, their noxious properties may be 

 greatly subdued. 



(205.) Jtesins. This class contains certain sub- 

 stances separated from the proper juice by some pro- 

 cess of secretion ; and not having any peculiar channels 

 appropriated to their reception, they form cavities and 

 force passages for themselves in the cellular tissue. Oc- 

 casionally they exude from the surface of the stem ; but 

 this must be considered accidental and not the result of 

 any provision made for their excretion, as is the case 

 with some substances which exude from certain glands 

 on the surface. 



(206'.) Oils. There are two classes of oils secreted 

 by plants : the one contains the highly volatile or essen- 

 tial oils as they are termed, which impart the fragrant 

 or disagreeable odours peculiar to different plants ; and 

 the other the fixed oils, such as those extracted ^ from 

 the fruit of the olive, the seeds of flax, &c. 



(207.) Volatile Oily. The first kind are gener- 

 ally contained in spherical or oblong cells in the leaves 

 and cortical parts of plants ; when held to the light 

 these parts appear as if they were punctured, owing 

 to the superior transparency of the receptacles in 

 which the oil is deposited. The St. John's-wort 

 (Ifi/jH-rii-uin /ifrfnrtitiiin) and any of the myrtle tribe 

 are familiar examples of this fact In the Umbellifera: 

 the oil accumulates in oblong club-shaped receptacles, 

 termed " vitta?," which are placed between the coats of 

 the seed-vessel ; and it is remarkable that their num- 

 ber and general appearance is so constantly the same 

 for each separate species that important generic cha- 

 racters are derived from this circumstance. 



(208.) Camphor is deposited upon the evaporation of 

 certain volatile oils, especially those extracted from some 

 of the Labiata?, as the common lavender. 



(20<>.) Fixed Oils. These are rarely found in the 

 cortical parts like the others, but are for the most 

 part extracted from the seed or its envelopes, and 

 sometimes from the pericarp, as in the olive. In 



