3io Kisch.—The Physiological Anatomy of 
cells. Such zones have been found in specimens of L epidodendron selagi - 
noides , Hickii , brevifolium , Wunschianum , fuliginosum , Scottii and sp., 
Sigillaria elegans i scutellata , &c. ; but, as their occurrence is not at all con- 
stant, it is possible that they are to be accounted for by events incidental to 
penetration and petrifaction of the tissues. PI. XXIV, Fig. 3, shows these 
zones in L epidodendron Scottii , where bands of black carbonized cells 
alternate with bands in which the structure is still preserved. 1 
(#) Zones caused by differences in size of the cells. In Sigillaria scu- 
tellata and allied species there are fairly continuous zones at rather irregular 
intervals, caused by a few layers of cells having their tangential walls 
closer together than the rest of the periderm (PI. XXIV, Fig. 4). Arber and 
Thomas considered them definite rings of growth, supporting their sugges- 
tion that the phellogen was of a periodic nature. 2 Of the zones described 
Text-fig. 23. Lepidodendron selaginoides. Zones in the periderm. A, zone formed by thin- 
walled distorted cells. U. C. L. Coll., A 9. B, zone formed by breaking down of cell- walls. 
U. C.L. Coll., A 01 23. x 265. 
they seem to be the most likely to have been formed at periods of diminished 
activity. The secondary wood, however, shows no sign of seasonal change, 
but it is possible that the nearness of the phellogen to the surface would 
make it more susceptible than the cambium to slight, unfavourable altera- 
tions in atmospheric conditions. 
(c) Zones formed by regular or irregular lines of cells which may be 
crushed or completely disorganized , or which may show signs of recent 
divisions . Under this heading have been grouped the various types of 
zones found in the periderm of Lepidodendron selaginoides , Wunschianum , 
Stigmariae, &c. In Lepidodendron selaginoides they form irregularly con- 
centric but not continuous lines, and consist of a few layers of cells, rather 
1 Gordon ( 12 ) suggested that these zones were due to some sort of periodic rest. 
2 Arber and Thomas ( 1 ), p. 141. 
