322 
F. Summers. 
pitted elements. The medullary rays contain secretory cells, while 
idiohlasts with mucilage occur in the hast. Surrounding the median 
nerve is a ring of collenchyma composed of small elements. This 
becomes discontinuous near the leaf-base and forms there two 
isolated patches (Fig. 24). In this region latex tubes,—the “ unge- 
gliederte Milchrohren” of Pax, are found in large numbers. 
Between the xylem and the collenchymatous sheath isolated strands 
of wood-parenchyma pass. These are three in number near the 
base, but die out near the apex. 
Ground-Tissue .—The palisade-tissue is two-layered and com¬ 
posed of large cells. The arrangement of these is quite regular, 
until they pass into 
those forming the 
middle collenchy¬ 
matous patch. Here 
they become some¬ 
what irregular (Fig. 
24). Under the sto¬ 
mata schizogenous 
intercellular spaces 
are found between 
two previously con¬ 
tiguous palisade- 
cells. ; Some of these 
cells assume a se¬ 
cretory function, 
especially near the 
leaf-margin (Fig. 
21), where the pal¬ 
isade-layer passes 
into a definite hy- 
poderm one cell in 
thickness (Fig. 22). 
This was not no¬ 
ticed by Herbert, 
probably because of 
the nature of the 
material examined. 
The spongy tissue 
Fig. 23. Series of elongated cells in mesophyll with associated secretory cells. 
