242 
Journal of Agricultural Research 
Vol. XIV t No. 6 
they are small and few in number. Here differentiation of the phloem 
seems to be in "advance of that of the xylem, or at least it has kept pace 
with the latter, whereas in the growing tip of a sprout no phloem is 
found when the first protoxylem elements have become evident. Even 
at these early stages collenchyma is present. This tissue is obviously 
here as a supporting structure, since in the underground sprout such 
tissue is not found. An endodermis has also become visible about this 
time. The cells are recognized by their regularity, smaller size, and 
starch content; Casparian strips are not developed. 
The other tissues develop in the same sequence as that described in 
detail for the sprout. 
THE LEAF 
The petiole and leaf blade may be considered as morphologically a 
lateral expansion of the stem, and their tissues as continuous with and 
derived from the latter. A good discussion of the gross morphology of 
these organs is found in De Vries ( 8 ). The substance of this is given 
here, and is followed by the writer's observation on the structure and 
development of the internal parts. 
The leaf primordia appear on the vegetative cone as small protuber¬ 
ances which soon push farther out and curve slightly, the adaxial surface 
becoming slightly concave. Continuing in this increase and curve, the 
primordia soon bend over the growing tip, their form thus constantly 
changing. This cone-shaped growing point, made up of the primordia, 
bulges on two sides, and soon these swellings can be recognized as the 
future blades of terminal leaflets. In a very early stage the terminal 
leaflet consists of a short petiole, a heavy midrib, and blade halves 
folded together adaxially. For a long time the terminal leaflet is far 
in advance of the other organs in its development. The latter differ¬ 
entiate only gradually, keeping pace with the elongation of the stem, 
and thereby providing space for the primordia of the lateral leaflets. 
The difference in rate of development is so great that the terminal leaflet 
is already i cm. in length and has become green long before the rest of 
the leaflets appear. The development of the lateral leaflets takes place 
in basipetal succession; consequently the uppermost leaflets are already 
green and well grown when the lowermost are still primordia. These 
lateral leaflets appear as protuberances on the petiole in the same man¬ 
ner as the primordium of the leaf itself appears on the vegetative cone. 
These are at first quite small, but soon elongate and pass through the 
same stages as do the halves of the terminal leaf blades. 
The first leaf hairs appear very early, developing acropetally on the 
convex surface of the primordium; later they are formed also on the 
inner surface in the same order. The hairs appear in two longitudinal 
rows on the veins, but none are found on the lamina itself. At first 
only glandular hairs are formed, but soon simple stiff hairs also develop. 
Both types increase rapidly in number and are mature long before the 
