R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 285 



base sunk in the top of a projecting many-celled papilla, in which 

 is secreted the irritant material (supposed to be formic acid) which 

 flows into the puncture produced by pressure on the hardened point, 

 the little terminal knob having first been broken off. 



Prickles consist of a somewhat elongated complex of epidermal 

 cells, frequently lignified, which, at the point, runs out into a single, 

 strongly-thickened, usually curved, sharp-pointed cell. 



Illustrative Figures.— Plate XIV. 



1. — Part of a transverse section through the growing point of a Fig-tree. X 1200. 



2.— Part of a transv. section of a branch of Black Currant, showing cork forma- 

 tion. X 500. e, epidermis. &, bast cells, p c, bark parenchyma. P, entire 

 product of the phellogen p 'g. c, radially arranged cork cells, arising centri- 

 fugally from the phellogen. y d, phelloderma, arising centripetally from the 

 phellogen. 



3,— Transverse section through the leathery cork of the Birch. X 660. 



4. — Vertical section through a stoma of Hellehorus viridis. s t, stoma, e, epi- 

 dermis, r, rind of leaf, a, air cavity. 



5. — Section through the under cuticle of Oleander, with the stomata, s t, lying in 

 a hairy pit. e, epidermis cells, p, parenchyma of leaf. 



6.— Vertical section through the marginal part of the thallus of MarcJiantia 

 polymorplia. p, colourless reticular-thickened parenchym cells, e, epi- 

 dermis of upper side, c, chlorophyllose cells, s t, stoma, w, partition wall 

 of an intercellular cavity, u, epidermis of under side, with dark- coloured 

 walls. 



7.— Sting of Nettle. X 75. 



