164 LENTICULAR GLANDS PRICKLES. [BOOK i. 



Naturelles.) In Tree ferns it is hardly possible to doubt that 

 the tubercles so common on the surface of the trunk, are the 

 points of roots either prepared for development, or arrested 

 in their growth by the dryness of the air that surrounds 

 them; for we find (in Dicksonia arborescens, for instance) 

 that the part of the stem which is next the ground is covered 

 with roots, and the part above it, surrounded by drier air, is 

 covered with tubercles. But it is not impossible that the 

 lenticular glands of the stems of ordinary trees, and the 

 tubercles of Tree ferns are different bodies, although so similar 

 as to be confounded under one name. 



It is extremely doubtful whether true lenticular glands are 

 any thing more than portions of the epiphlceum, disorganised 

 by some unknown power. Mohl states that they are found 

 in the epiphlceum, that is, between the epidermis and the 

 mesophloeum, and consist of greenish or colourless (or in 

 Berberis yellow, and Sambucus red) cells which lie in rows 

 perpendicular to the axis of the branch, and united towards 

 the interior with the mesophlo3um. He considers them a 

 partial formation of cork.* Unger compares the true len- 

 ticular glands to the soredia of Lichens, and the reproductive 

 granulations of Scale Mosses ( Jungermanniacese) ; and he 

 considers them in some way connected with the respiratory 

 process : even as obliterated respiratory organs. Meyen 

 regards them, not as obliterated respiratory organs, but as 

 formations intended to maintain an air communication 

 between the exterior rind and the new green bark of trees ; 

 for he says that the tissue of old bark is so compactly com- 

 bined as to cut off all direct communication between the air 

 and the cavernous parenchym of the green bark. 



7- 



6. Of Prickles. 



PRICKLES (aculei) are rigid, opaque, conical processes, 

 formed of masses of cellular tissue, and terminating in an 

 acute point. They may be, not improperly, considered as 



* I take this from Taylor's Magazine, xii. 58, where there is an (imperfect ?) 

 translation of Meyen's report on this and other subjects. Mohl's original paper 

 I have not seen, and the translation is in part unintelligible. 



