632 CONSERVATIVE ORGANS. [LECT. XI. 



are more easily detected than the internal, and 

 appear to be more numerous. They may be ar- 

 ranged under two genera, the sessile and the pe- 

 diculated; and each of these subdivided into 

 species. 



a. The sessile gland lies on the surface of 

 the stem, or of the leaf, or is slightly depressed in 

 the cutis. It comprehends three species : the 

 simple papillary, the compound papillary, and the 

 scaly glands. 



1. The simple papillary gland, glandula pa- 

 pillaris simplex, is usually situated on the lower 

 disk of leaves; and, in many instances, it appears, 

 to the unassisted eye, a mere pore, exuding a drop 

 of viscous or oily fluid ; but is, in fact, a small 

 tubercular elevation. Thus on the back of the leaf 

 of Crescent-leaved Passion-flower, Passiflora la- 

 nata, the dark spots seen by the naked eye on each 

 side of the midrib, are found to be slightly elevated, 

 circular, papillary glands, with an apparent pore in 

 the centre (Plate 9, fig. 27. a.) when viewed with a 

 good glass ; but when the gland is placed under 

 the microscope, the pore is discovered to be a de- 

 pression only, covered with a very thin transparent 

 epidermis, which extends over the whole surface 

 of the gland. In general, however, the papillary 

 glands are more conspicuous ; and appear, even 

 to the unassisted eye, small elevated bodies, with 

 a broad base, placed, in some instances, as it were, 

 in a socket. Their structure is cellular; but the 



