115 SECONDARY THICKENING. 



as those in the stem, but are often surrounded by a layer of thick 

 walled fibrous tissue, which adds much to their strength. 



Treat a small thin leaf (Oxalis), with KOH and mount. The 

 preparation may be made permanent by washing thoroughly with 

 water and mounting in glycerin jelly. 



OBSERVE: 1. The anastomosing Fibro-Yascular bundles, 

 indicating their full development. 



2. The free ends of the bundles, consisting of the spiral 

 tracheids, in close contact with the mesophyll cells of the leaf. 



3. The appearance of one of the larger bundles in tran- 

 section. 



LENTICELS. 



Illustration: Elliptical wart-like thickenings on the stem of 

 ELDEB (Sambucus Canadensis}, or Moox SEED VINE (Menispermum 

 Canadense). 



PREPARATION FIRST: Fasten pieces of the stem in the jaws of 

 a hand microtome, and make several thin transections through a 

 lenticel. Mount in water and OBSERVE : 1. The ruptured epider- 

 mis of the stem. 



2. Beneath this the true cork cells, not closely united but pro- 

 vided with inter-cellular spaces, which allow free communication 

 between the inner tissue and the air outside. 



3. Below the loose cork cells, a layer of thin- walled rectangular 

 ones, constituting the true meristematic region of the cork, the 

 Phellogen. The continued development of this tissue produces in 

 time a ring of cork which breaks away from the stem in the form 

 of the rough corky bark. 



4. Inside of the Phellogen and formed from it is the Phel- 

 loderm, a layer of cells regular in outline, containing protoplasm, 

 and in many cases chlorophyll. These cells resemble those of the 

 primary cortex, and are formed to replace any of the cortical tissue, 

 that may have been obliterated by the pressure resulting from the 

 rapid growth of firmer tissue beneath. 



Vines' Text Book of Bot., p. 212; Goodale, p. 151: DeBary, 

 p. 560; Strasburger, p. 153. 



