Leaf and Sporocarp in Mar si Ha quadrifolia , Z. 123 
though not with strict propriety, call these layers dermatogen 
and periblem, and the pericline itself a dermatogen-wall 
(d. w., Fig. 7, d.,pb., Fig. 22). 
The dermatogen soon splits by another pericline into 
hypodermal and epidermal layers (Figs. 8-10, 22), each of 
which remains of one cell in thickness even at maturity, 
though numerous anticlines, both longitudinal and transverse, 
are formed in each. On the line of the median wall, of each 
section-wall and of the halving anticline of section I, there 
are formed intercellular spaces (a. } c. } Figs. 8, 22) between the 
periblem and hypodermis, which are the beginnings of the 
fourteen (primary) longitudinal air-canals of the petiole 
(a. c., Fig. 9). 
The single periblem-cell of each half of section I cuts 
off by a pericline at the inner end a second portion of plerome 
(pi., Figs. 8, 22). Then each of the remaining periblem-cells 
of section I, the single periblem-cell of each of the other 
sections and that of the marginal cell, divides by a pericline 
into an inner and outer cell (Figs. 8, 22). Of these the inner 
cell divides by anticlines and periclines to form the loose 
mesophyll-tissue of the mature petiole (mp., Figs. 8-10), 
while the outer cell gives rise to both the longitudinal and 
transverse partitions between the adjacent air-canals (p. c., 
Fig. 8). These latter cells swell in the middle (as seen in 
cross-section of the petiole) and grow out at the ends into 
papilla-like tips (Figs. 8, 11, 12), touching their fellows of the 
adjacent sections, but leaving an intercellular space sur- 
rounding each tip. The tips thus formed are soon cut off 
by longitudinal anticlines (Figs. 8, 11), forming a pair of nearly 
isodimentional cells (c. p. c., Figs. 8, 11) in each air-canal, 
opposite each primary partition-cell, of which there are 
usually eight in the length of each segment. From these 
eight pair of cells are developed the eight transverse partitions 
of the air-canal in each segment. These remain one cell 
thick even at maturity, but during their later development 
many intercellular openings or pores are formed, allowing the 
passage of air through them ( c. p. p ., Figs. 9, 10, 13). 
