260 



STRUCTUKE OF STEMS AND ROOTS 



N^ 





431. Usually very early in the life of the stem a corky 

 layer of bark is produced. This is the product of an active 

 layer of cells called phellogen. This layer is first found 



at those places 

 where the stomates 

 or breathing pores 

 were located. The 

 epidermis is first 

 crowded off at these 

 places, and the 

 rough corky spots 

 are called lenticels. 

 Phellogen is very 

 active in the cork 

 oak of Spain, but 

 we find it in nearly 

 all wood} r plants. 

 In such plants as 

 button wood (syca- 

 more), in which the bark peels off in thin, flat layers, 

 the phellogen layer is nearly uniformly active in all 

 parts, while in many other cases there is very little unifor- 

 mity. In wahoo (burning bush) it is in four bands, giving 

 rise to four corner wings. In the section of menispermum 

 already studied, it is found only under the lenticel spots 

 where the stomates have been located. 

 Fig. 409 shows structure of the outer 

 bark as found in the whole circum- 

 ference of the three -year -old stem 

 of red currant. To study phellogen 

 and corky tissue : Cut thin cross - 

 sections of red currant from 

 stems two or three years old which have been kept in 

 alcohol at least several hours. The sections should be 

 stained. With the highest power make a careful study of 



408. Section of basswood stem, 5 years old. 

 The cone-shaped growths of phloem are plainly seen. 



P 



.'/ 



409. Cross-section of red cur- 

 rant twig, showing bark, 

 c, corky tissue; p, phellogen; 

 (/, parenchyma or cortex. 



