294 



STRUCTURE OF STEMS AND ROOTS 



off at these places, and the rough corky spots are called 

 lenticels. Phellogen is very active in the cork oak of Spain, 

 but it occurs in nearly all woody 

 plants. In such plants as button- 

 wood (sycamore) , 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 uni- 

 formity. In wahoo (burning-bush) it is in four bands, giv- 

 ing rise to four corner wings. In the section of menisper- 

 mum already studied, it is found only under the lenticel 

 spots where the stomates have been located. Fig. 465 shows 

 structure of the outer bark as it occurs in the whole circum- 

 ference of the three-year-old stem of red currant. 



499. To study phellogen and corky tissue: Cut thin cross- 



465. Cross-section of red cur- 

 rant twig, showing bark. 

 c, corky tissue; p, phellogen; 

 g, parenchyma or cortex 



466. White pine stem in radial longitudinal section. 



Tracheids on the left with medullary rays crossing them. Next to the wood 



is the phloem, then fundamental tissue, then the dark bark. 



sections of red currant from stems two or three years old 

 that have been kept in alcohol at least several hours. The 

 sections should be stained. With the highest power make 

 a careful study of the phellogen and the corky tissue outside 



