124 Johnson. — O71 the Development of the 
The portion remaining of each primary partition-cell (l.p. c., 
Fig. 11) grows in a radial direction and splits by periclines, 
while it at the same time grows in the direction of the length 
of the petiole and divides by transverse anticlines. Thus are 
formed the longitudinal partitions between the adjacent air- 
canals, which are also one cell in thickness (Figs. 9, 10, 13). 
As each of these primary longitudinal partitions elongates 
with the lengthening of the petiole, it is seen (/. p. c., Fig. 12) 
that the primary cross-partition cell at one end is nearer the 
upper wall and that at the other is nearer the lower wall. 
Then when the first transverse anticline is formed it is some- 
what oblique and forms thus two wedge-shaped cells, each 
with a cross-partition cell at the broad end and none at the 
narrow one (/. p. c ., Fig. 12). The cross-partitions in adjacent 
canals are thus alternate. These wedge-shaped cells continue 
to elongate and divide by transverse anticlines (Fig. 13) till 
in the mature petiole the cross-partitions are far apart. Here 
again in the longitudinal partitions we find at maturity many 
small intercellular openings or pores, the ‘ meats ’ of Meunier 
(l-P-P; FigS. 10, 14). 
When the epidermal surface of a section is two cells broad 
and four cells long (Fig. 15), there is cut out of each cell, 
by a semicircular anticline at the upper end, a small cell 
which gives rise to one of the numerous trichomes that clothe 
the young leaf. The rest of the epidermal cell then divides 
further by anticlines (Fig. 15), and more trichomes arise in 
the cells thus formed, while the epidermal cells at maturity 
become much elongated (Fig. 16). Each trichome-cell grows 
out beyond the surface of the epidermis, and swells to a knob 
at the outer end ( tc ., Figs. 2, 3, 9), which soon elongates in 
the direction of the length of the petiole. On the lower or basi- 
scopic side it projects but little (Fig. 3), while it grows out 
toward the apex of the leaf to the long multicellular hair ( tc ., Fig. 
17) that is supported by the basal or stalk-cell which remains 
wedged in between the epidermal cells ( b . c. tc ., Figs. 9, 16, 17); 
later in the development most or all of these trichomes are 
cast off, and the petiole thus becomes naked at maturity. 
