56o 



SECONDARY CHANGES. 



5 The phenomena described usually extend uniformly over the whole of th^ 

 stem and its branches; examples, however, occur of differences in the formation of 

 periderm in zones at different heights. This is the case in Pinus sylvestris (p. 556), 

 a tree which is distinguished by the thick bark on the lower stem, and the fine peehng: 

 bark on its apex and branches, from its allies, e.g. P. Laricio, in which the thick 

 bark extends to the apex. According to Hartig this is also the case in the Birch, in 

 which the higher portions of the stem, and the branches, independently of their age, 

 constantly remain covered by superficial periderm only. 



Sect. 179. Leniicels \ In most woody plants which form periderm the uniform 

 peridermal integuments hitherto described are interrupted at definite points, to be 

 more exactly indicated below, by bodies which are as it were inserted in them and 

 belong to them, to which De CandoUe gave the name of Leniicels, and Du Petit- 

 Thouars the more descriptive one of corlical pores. Only in relatively few woody 

 plants, provided with a regularly repeated annular formation of bark, have lenticels 

 as yet been sought in vain : Vitis vinifera, Lonicera italica, L. periclymenum, Tecoma 



°Fimm 



fc-^^"^*oC- 



FIG. 221.— Transverse section through the cortical surface of a shoot of Betula alba one year old (145). e, e 

 epidermis, s stoma, d glandular scale, ji—p superficial periderm, with a lenticel interpolated below the stoma. 

 In the lenticel two firmer, denser tangential bands are visible ; but the narrow intercellular spaces containing 

 air are left undrawn in the whole of the lower part, on account of the low magnifying power. 



radicans, species of Clematis, Philadelphus and Deutzia, and Rubus odoratus, while, 

 on the other hand, they appear in other plants, which are nearly related systematically 

 to those mentioned, or agree with them in growth, habit, and formation of bark ; as 

 in those Loniceras which are not climbing, Solanum Dulcamara with annular bark, 

 Ampelopsis, Periploca, Wistaria sinensis, &c. 



Those species which form lenticels anywhere, have them both on the stem and 

 its branches, and on the root. 



According to its structure (comp. Fig. 221, and Figs. 222 and 223 below), the 

 lenticel may in most cases be appropriately termed a local, bi-convex swelling of the 

 periderm, often projecting above the surface as well as internally; it is distin- 

 guished from the rest of the layer by the presence of narrow intercellular spaces 



1 Von MoU, Unters. iib. d. Lenticellen, Verm. Schriften, p. 233; also p. 229. — A. Trecul, 

 Comptes Rendus, torn. 73, p. 15. — E. Stahl. Entwicklg. u. Anatomic d. Lenticellen, Diss, and Botan. 

 Zeitg. 1873.V-G. Haberlandt, Beitr. z, Kenntn. d. Lenticellen, Wiener Acad. Sitzgsber. Bd. 72 (1875). 

 [Also Kreuz, Entw. d. Lenticellen von Ampelopsis hederacea, Wiener Acad. Sitzgsber. Bd. LXXXIII. 

 1881.; and Potonie, Lenticellen d. Marattiaceen. Ref. Bot. Centralbl. 1881, Bd. 8, p. 70.] 



