158 



ACADEMIC BOTANr. 



386. Cortex (Cork), or 



A.C./f\.. 



Fig. 230. — Sections of a stem uf the 

 Lace-bark-tree (Lagetla Uiitearia) of the 

 "West Indies, showing one of the lace- 

 like layers of the liber, the outer layers 

 still unopened and cut squarely off; 

 c, cambi\ini layer. Corky bark (outer) 

 very thin and smooth. 



Outer, Bark is purely cellular, 

 consisting of empty cells, which 

 are small, cubical, flattened, 

 usually colorless, and always 

 impervious to water. Ordi- 

 narily, it increases for a fe^^ 

 years only; but in the Cork 

 Oak of Spain (Fig. 229) it con- 

 tinues to grow from year to 

 year, and is highly developed, 

 furnishing the staple which 

 gives its name to the tree. 



387. Cork is first cut when the tree 

 is 25 yeai-s old (Frontispiece, Lesson I. ) . 

 The tree is then left untouched for 8 

 or 10 years, for the cork to he re- 

 newed, when the harvest or cutting is 

 repeated. This process continues at 

 like intervals for a hundred and fifty 

 years, the trees producing good cork 

 for that period. The cuttings do not 

 injure the trees, because the living 

 parts a,re not disturbed. 



388. Surface - bark. — When 

 the corky bark ceases to grow 

 its cells ai;e no longer active. The continued growth of 

 the wood and liber therefore stretches the corky bark until 

 it splits into seams, and forms a surface-bark which is 

 characteristic in each species. In the oak and pine the 

 seams are longitudinal and the cork persists, its surface 

 becoming blackened. In the plane (sycamore) and birch 

 the cork splits both longitudinally and horizontally, falling 

 off in plates. 



389. The bark sometimes interrupts the usual form of the wood. In 

 the Cross-stem (Bignonia capreolata) three or four wood-circles are 

 deposited, and then an extraordinary development of cellular tissue 

 takes place in the proper liber-cells next to the cambium ; this tissue 

 usurps the wedge-form itself, and throws the wood-layers into four 

 rectilinear shapes, so that the wood, in transverse section, has the 

 form of a Greek cross, from which the stem gets its common name. 

 In spring the bark is easily removed, leaving the four angles finely 

 exhibited. 



