LECT. XI.] CAULINAR AND FOLIAR APPENDAGES. 631 



Citrus, &c. A beautiful variety of this gland 

 is observed on the leaf of Coffea AraUca, si- 

 tuated in the angles formed by the parting of the 

 larger costse from the midrib : its follicle, which 

 occupies a space rather greater than the thick- 

 ness of the leaf, forms a small elevation on the 

 upper disk, and opens on the lower by a large ex- 

 cretory pore, guarded by stiff hairs inclining over it. 



The real glandular part of the follicular gland 

 is cellular, and forms the parietes of the fol- 

 licle, which is the receptacle only of the secre- 

 tion; and this, in general, is an essential oil. As 

 the odours of leaves depend chiefly on the exhala- 

 tion of their essential oil, they are often regu- 

 lated by circumstances affecting the excretory 

 ducts of these follicles. Thus the duct being closed 

 by the pressure of the cells turgid with sap, in the 

 fresh stem and leaf of Sweet-scented Vernal 

 Grass, Anthoxanthum odoratum, no odour is per- 

 ceived ; but it opens, when these cells shrink, as 

 the grass dries, and, then, the agreeable perfume 

 which is peculiar to new hay is exhaled. The 

 odour is permanent in some plants, as Mint, Sage, 

 &c. but very evanescent in others, as, for example, 

 in the leaves of Gaultheria odorata, which yield a 

 very agreeable odour when fresh, but become 

 scentless a few hours after their separation from 

 the tree. 



B. The EXTERNAL caulinar and foliar glands 



S 8 4 



