282 R. BRAITIIWAITE ON THE HISTOLOGY OF PLANTS. 



phellogen which give the thickness to the periderm, but parenchym 

 cells containing chlorophyl are also formed ; these, however, are 

 always the daughter cells of the phellogen lying on the inner side 

 which l5.ecome thus metamorphosed, and constitute what Sanio terms 

 the Phellodermrij very well seen in the currant tree. 



Bark. — x\fter production of more or less numerous cork lamelL^, 

 the phellogen dies or loses its vital activity, but a development 

 of secondary cork tissue takes place within the bast part of the 

 vascular bundle, in the form of tangential rows of tabular cork 

 cells, which loosen from the growing outer part of the vascular 

 bundle. The cork lamellae, as it were, cut out and force off from 

 the rind, flat pieces in form of scales or rings ; all this outer part 

 is dead, and the process oft repeated from the circumference of 

 the stem, causes the new cork lamellse to become gradually em- 

 bedded more deeply in the growing cortical tissue, and we get a 

 constantly thickening peripheral layer of dry tissue separating 

 from the living part of the rind ; this is the bark. The condition 

 is very evident in the large scales of bark in Platanus orientalis 

 or sycamore, and in old stems of the Pinus silvestris or 

 Scotch pine, and in the ring-like bands of the cherry tree. In 

 the oak, lime, poplar, elder and horse-chestnut similar plates of 

 thin-walled cells arise in the interior of the bast bundles, but the 

 old dried scales do not fall off, but tear only at the margins in a 

 longitudinal direction, so that the stem becomes clothed with bark 

 consisting of several dead scales lying under each other, presenting 

 internally all the elements of bast, and externally primary cork 

 tissue. In the pine and larch we have a fissured periderm, like 

 that of the horse-chestnut, and in the pine consisting partly of 

 thin-walled and thickened cells in alternate layers, but the conifers 

 are specially remarkable for the pi'esence of a spurious large celled 

 parenchym tissue, which appears between the periderm layers and 

 separates the elements of the bast bundle into smaller or larger 

 groups. 



Lenticels. — These are due to a peculiar local cork formation, and 

 appear as little roundish spots on the annual shoots of treeS; while 

 the epidermis continues uninjured, and before tlie periderm is formed. 

 In the second summer the epidermis splits longitudinally over the 

 lenticels, and they form more or less prominent warts, which by a 

 median furrow frequently become bilabiate ; their surface is mostly 

 brown,, and their substance to a certain depth dry and corky. By 



