232 
Journal of Agricultural Research 
Vol. XIV, No. 6 
detailed arrangement of these tissues, the condition found in the stem pre¬ 
vails (Pl. 36, A). The protoxylem is endarch, its first elements consisting 
of loosely ringed and spiral cells which are gradually superseded by larger 
closely ringed or spiral ones. Toward the outside, reticulate and porous 
vessels are found. Between the vessels, which are arranged in radial 
series, are uni- and bi-seriate medullary rays and small tracheids. The 
inner and outer phloem groups are in form and structure similar to those 
of the stem and will not be treated further. Throughout the petiole a 
cambium is developed which gives rise to some secondary growth (Pl. 
36, A). The elements formed by the cambium are mostly vessels and 
tracheids, six or eight rows representing the extent of development by 
this meristem (PL 43, C). 
The midrib projects both above and below from the surface of the 
lamina. On the lower side it is prominent and convex in outline. On 
the upper side it forms an indistinct flat ridge which is only noticeable 
in cross section of the leaf. As seen in cross section, the vascular tissue 
of the midrib, like that of the petiole, forms a semicircle, the open side 
toward the upper surface of the leaf. But here the vascular tissue is 
not at all, or only partially, broken into individual strands. The cortical 
tissue is differentiated near the epidermis to form a layer of collenchyma. 
This layer of collenchyma is rarely more than two cells thick except in 
the upper projecting ridge, which is composed almost wholly of this type 
of tissue. A cambium is rarely developed. The cells between the xylem 
and the outer phloem are parenchymatous and sometimes of rather uni¬ 
form arrangement, closely resembling those of the cambium. Old ma¬ 
terial shows that these cells may give rise to some weak secondary growth, 
appearing most prominent at the base of the leaf, gradually disappearing 
along the rachis, and not extending to the terminal leaflet. 
Both in petiole and midrib large ovoid parenchymatous cells are found 
between the external phloem groups. These cells are always present, 
though in varying amounts; their significance could not be determined. 
The lateral veins are similar to the midrib in anatomy and morpho¬ 
logical structure. The projecting ridges become reduced, and the amount 
of collenchyma is limited to one layer on the lower surface. The vascular 
tissue also decreases gradually with the size of the vein. The phloem 
groups become rarer, and the cells of each fewer. The xylem also becomes 
reduced till finally the terminal branchlets consist of one or two spiral 
elements and conducting parenchyma. 
The mesophyll of the lamina consists of a palisade layer and spongy 
parenchyma (PL 36, E). The palisade tissue, which lies on the upper 
side, consists of a single layer of elongated and closely packed cells in 
uninterrupted contact with one another, except in mature leaves, where 
they are sometimes separated by narrow intercellular spaces. The 
palisade layer abuts upon the spongy parenchyma, the cells of which 
are irregular, loosely arranged, and poorly provided with chlorophyll. 
