36 ARISAEMA TRIPHYLLUM 
differentiation of certain cells (TExT FIG. 38, A) of the middle 
layer to form the primary vascular elements 
occurs first. The epidermis is clearly 
marked next, followed by the formation 
of the palisade cells by the top of the three 
inner layers and the formation of the 
spongy parenchyma by the lowest layer 
and the remainder of the middle layer. 
There is practically no variation in leaf 
structure coincident with changes in con- 
ditions of growth. The petioles of plants 
grown in the shade are slightly longer than 
of those grown in full light, and the size 
of the blades may vary from year to 
year; but the change in thickness 
due to increased palisade formation found 
in leaves of many plants following change 
from weak to strong light, is not at all 
Fig. 39. Surface view evident. A slight increase in the number 
of lower epidermis. AA, 
am 
LS 
Were 
of stomata, and the occasional appearance 
accessory cells. 
Fic. 40. Surface view of a few stomata on the top of leaves of 
of upper epidermis. shade plants has been noted. So far, 
IG. 41. Twin stomata 
ak er eae: however, experiments have failed to show 
whether these changes are the result of 
different conditions or merely indicate individual variation. 
STRUCTURE OF PETIOLE AND SCAPE 
The vascular system of embryo, root and leaf are treated in 
these sections. The similarity of structure of petiole and scape 
make possible a common description. These bodies are composed 
of a peripheral layer of epidermis with slightly thickened walls, 
and two or three underlying layers of small parenchymatous cells. 
Inside this peripheral portion is a circle of well developed vascular 
bundles, each with a strong strand of collenchyma separated by 
one layer of cells from the epidermis (TEXT FIGs. 43, 44). The 
vascular elements of these bundles consist of a few spiral ducts 
and phloem elements, and are duplicates of the principal veins 
of the leaves. The inner portion of both petiole and scape is 
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