290 
Kisch . — The Physiological Anatomy of 
Watson noted that in Lepidodendron Hickii the phellogen was not yet 
formed all the way round, though part of the periderm was twenty cells 
thick ; 1 while in a specimen of Lepidodendron intermedium there is an arc 
of secondary tissue which is thirty-five cells thick in the centre, but formed 
only on one side of the stem (Text-fig. 3). 
Text-fig. 3. Diagrammatic transverse section of Lepidodendron intermedium , showing unequal 
development of periderm,/^; st, stele. U. C. L. Coll., A 48. x 12. 
(3) Nature of the Meristem. 
The phellogen arises by tangential divisions in certain of the cells 
of the outer cortex. Its formation, however, is not quite the same as 
Text-fig. 4. Lepidodendron selaginoides. Transverse section from the outer cortex showing the 
formation of the phellogen. Will. Coll., 342. x 265. 
is commonly found in recent plants, as it does not as a rule form a single 
continuous layer of meristematic cells, the products of which all pass 
directly into permanent tissue. The most regular case found is that of 
1 Watson ( 32 ), p. 9. 
