108 THE STEM 



in many species of shell-shaped layers of periderm which enclose 

 masses of dead or dying phloem tissue (cf. Eames and Mac- 

 Daniels, p. 212, Pig. 96). 



The first-formed as well as later developed periderm layers 

 in stems are usually provided with aerating structures termed 

 lenticels. As stated above, lenticels are usually initiated by the 

 appearance of a phellogen beneath a stoma (cf. Eames and ]\Iac- 

 Daniels, p. 219, Fig. 100). In the development of a lenticel, the 

 phellogen, instead of producing typical cork, forms a mass of 

 loosely-arranged cells with unsuberized walls which make up the 

 complementary tissue. This tissue in many lenticels may be sub- 

 divided by layers of smaller more compact cells which are termed 

 closing layers. The pressure exerted by the outwardly-developed 

 mass of complementary tissue is sufficient to rupture the epi- 

 dermis which, together with the underlying layers of adjacent 

 cork, curls back from the edges of the lenticel as flaps of broken 

 tissue. In many plants (e.g., SamJ)ucus), the extruded comple- 

 mentary tissue is very prominent. According to De Bary (p. 561) 

 the i)uffy swelling of lenticels in trees during wet weather may 

 be the result of the "hygroscopicity " of the complementary 

 tissue. 



II. Material for the Comparative Study of the Stem. — The 



choice of material for the study of stem anatomy will depend 

 upon the forms available as well as upon the points to be illus- 

 trated. The following stem types have proved useful and are 

 recommended. Free-hand sections of stems stained with phloro- 

 glucinol and hydrochloric acid are of considerable use. l>ut for 

 the finer details of structure and development, permanent mounts 

 of critically stained sections are necessary.^ 



1. The si (III of the gcr<niii(ni {P<'](n-g(nilinu sp.). Examine a 

 transverse section of the stem, and sliulx" the following tissues 

 and regions from the edge of the section inwardly, viz.: 



{a) The epidermis, a uniseriate layer of small oval or elliptical 

 cells with thick inner and outer walls (the latter covered with a 

 thin cuticle) and somewhat thinner radial walls. A i)rotoplast, 

 which may appear somewhat collapsed, should be evident in most 

 cells. Certain of the epidermal cells have given rise to stomata 



1 Cf. Appendix, p. 140. 



