126 



BOTANY. 



their formation the cork-cells lose their protoplasmic con- 

 tents, while beneath them new cells are constantly being cnt 

 off from the cells of the generating layer ; in this way the 

 mass of dead cork tissue is formed and pushed out from its 

 living base. 



158. — The generating tissue is called the Phellogen,* or 

 Cork-cambium ; it occurs not only in the hypoderma, but in 

 any other part of the fundamental system, and, as will be 

 shown hereafter, in the secondary fibro-vascular bundles. 

 When a living portion of a plant is injured, as by cutting, 

 the uninjured parenchyma-cells beneath the wound often 

 change into a layer of phellogen, from ^hich a protecting 



mass of cork is then 

 developed. 



150. — Lenticels 

 are in many cases the 

 result of a restricted 

 corky growth just be- 

 neath a stoma. Phel- 

 logen consisting of a 

 few cells of the hypo- 

 derma, is formed im- 

 mediately below a 

 stoma (Fig. 113, x) ; 

 by the growth of cork 

 from this phellogen the epidermis is pushed out and finally 

 ruptured, exposing the roundish or elongated mass of corkf 

 (Fig. 113). Lenticels are of frequent occurrence on the young 

 branches of birch, beech, cherry, elder, lilac, etc., and may be 

 distinguished by the naked eye as slightly elevated roughish 

 spots, usually of a different color from the epidermis. 



(a) The examination of the tissues of the fundamental system may 

 in general be made with. considerable ease, by making transverse, tan- 

 gential and radial sections. 



* From the Greek (jieUoc, cork. 



f It appears quite certain that not all lenticels develop from the 

 hypoderma beneath stomata ; phellogen forms beneat'i the epider- 

 mis at other points, and gives rise to lenticels in a way essentially as 

 in the other cases. 



Fig. 113,— Transverse section of a portion of the 

 intemode of a youn? twig of Bctula alba, c, cuticle, 

 somewhat separated from the epideiTBis ; e, e, epider- 

 mis ; a, cavity under the stoma seen in cross-section 

 above ; i», x, cells which are beginning the process of 

 multiplication by fission, constituting the phellogen 

 of the future lenticel. X 375.— After De Bary. 



