158 PROGRESS OF MICROSCOPICAL SCIENCE. 



bundle; which become the mother cells, so that all the young cortical 

 tissue becomes pushed off by the cork tissue. 



In many cases it is not solely cork cells proceeding from the phel- 

 logen which give the thickness to the jjeriderm, but parenchym cells 

 containing chlorophyll are also formed ; these, however, are always 

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

 become thus metamori)hosed, and constitute what Sanio terms the 

 Phellodcrma, very well seen in the ciu-rant tree. 



Baric. — After jn'oduction of more or less numerous cork lamcUfe, 

 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 gi'owing outer part of the vascular bundle. The cork 

 lamellaj, as it were, cut out and force off' from the rind, flat pieces in 

 form of scales or rings ; all this outer jjart is dead, and the process 

 oft repeated from the circimiference of the stem, causes the new cork 

 lamellae to become gradually imbedded more deeply in the gi'owing 

 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 syhestris 

 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 fissiu-ed 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 pre- 

 sence of a sinirious large-celled j^arenchym 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 imiujured, and before the jjeriderm 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 sm-face is mostly 

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

 growth in thickness of the shoot, the lenticels become expanded into 

 transverse stria;, then cork or bark fonns and s^^lits the rind beneath 

 them, the bark scales off, and so they disappear. These and other 

 structures are very well illustrated in a plate which accomjjanies the 

 paper. 



