Notes on Bark Structure 143 



for a longer time and on Abies it remains often longer than the 

 third year. The phellogen forms in the second to fifth year 

 or even later in Cuprcssus, Sequoia and Taxus, and the epider- 

 mis remains longer and expands as the twig thickens. 



The differences in appearance and in the method of shed- 

 ding the plates and scales are due to the position and the 

 lateral extent of the cork-layers, or periderm. In general the 

 inner cork layers resemble in structure the superficial ones. 

 In Taxus, Sequoia, Abies, Taxodium, Cupressus, Thuja, Libo- 

 cedrus and Juniperus, the periderm is thin, while in Pinus, 

 Picea and Larix, it is thickened. The periderm in Abies cana- 

 densis takes up the coloring matter in alternate layers. 

 The phellogen remains active for more than one vegetative 

 period in Larix and so forms annual layers in its periderm: 

 this in other genera is rare. A sclerotic periderm is .almost 

 exclusively characteristic of Pinus. Excellent examples of 

 ring ' t borke ' ' are found in Thuja, Juniperus, Cupressus, Tax- 

 odium and Libocedrus; of scaly bark, in Taxus, Larix, Picea 

 and Pinus; and of bark which does not exfoliate, in Abies. 



Middle bark. A hypoderm of sclerotic fibers is found in 

 Cupressus, Thuja, Libocedrus, Picea and Sequoia; while in 

 Pinus and Larix it consists of sclerenchyma cells occurring 

 singly or in groups, around the periphery. 



Collenchyma occurs in a typical closed layer in 

 Picea, Abies, and Larix, while in the remaining conifers it 

 appears as a broken layer of regular sized columnar bundles. 

 Abies, Picea, and Larix are characterized by sclerotic cells scat- 

 tered and also in groups throughout the middle bark; gener- 

 ally they have fantastic shapes and the cells are considerably 

 enlarged. These sclerotic cells do not occur in Pinus, Sequoia, 

 Taxus, Taxodiutn, Cupressus, Thuja, Libocedrus nor Juniperus. 



The phelloderm takes part in the building up of the mid- 

 dle bark in a prominent manner when "bo<rke" formation is 

 late, but is nearly always absent from the inner periderm. 

 It is formed in Pinus and Taxus but always consists of a very 

 few rows of cells even in heavily developed cork layers. In 

 Pitnis, Larix and Picea, it becomes hardened (sclerotic). 



Crystals of calcium oxalate occur in various ways. They 

 are characteristic of Pinus in the aggregate form. In Libro- 

 are characteristic of Pinus in the aggregate form. In Libo- 



