146 Forest Club Annual 



b. Spindle form, mostly isolated bast-fiber-like stone 

 cells ; broad thin-walled cork layer with a narrow 

 layer of sclerotic cork cells. 



Larix 



JULIFLORAE.* 



Outer bark. In Salix the phellogen originates in the epi- 

 dermis, in the other genera of this group it originates in the 

 outermost layer of cells of the primary bark. On a year old 

 twig usually several layers of cork are present. The epider- 

 mis remains on not longer than the second or third vegetative 

 season while the superficial periderm renews itself for a longer 

 period of years, certainly through a decade, and is only excep- 

 tionally or perhaps not at all thrown off in Betula, Fagus and 

 Carpinus. 



Strong walled plate cork is formed in Betula, Alnus, Liqui- 

 danibar, Ostrya, Carpinus, Corylus; Quercus, Fagus and Cas- 

 tanea. Cubical and thin walled cork cells predominate in 

 Celtis, Ulmus, Morus, Platanus, Populus and Salix; all of these 

 except Ulmus form stone-cork plates. 



Middle bark. The characteristics of the middle bark are 

 found in the presence of secretion cavities and in the occur- 

 rence of sclerenchyma, and perhaps the most noticeable feature 

 is the never failing presence of calcium oxalate crystals in all 

 the genera of this group. 



In the occurrence of sclerotic parenchyma several modifi- 

 cations may be distinguished, of which three are given below : 



(a) The sclerenchyma accompanies the primary bast fibers 

 and forms a closed ring of mixed stone-cells and bast 

 fibers in Betula, Alnus, Ostrya, Carpinus and Corylus. 



(b) Besides the closed ring mentioned in (a), sclerotic 

 cells occur singly or in groups scattered throughout 

 the whole region of the middle bark in Celtis, Quercus, 

 Fagus, and Castanea. 



(c) The parenchyma does not become sclerotic in Ulmus 

 and in some species of Morus, Salix and Populus. 



The phelloderm takes a prominent part in building up the 

 middle bark in those species which have a persistent super- 

 ficial periderm, and a rather insignificant part in those which 

 do not have. Since the parenchyma of the middle bark does 

 not undergo an unusual tangential stretching, and the primary 



*Eichler's classification. 



