90 ELEMENTS OF PLANT ANATOMY. 
4. Origin and Comparative Anatomy of Leaf. 
Origin. — In the mosses it has already been shown that the 
leaf rises from a portion of each segment of the apical cell, which 
portion, first a single cell, afterward develops into a flat organ 
consisting of only one layer of cells and with no distinction of 
blade and petiole. In all plants higher than this the stem struc- 
ture is much more complex. The presence of bundles and their 
connection with the leaf, and the increased complexity of the 
leaf itself, render its exact manner of development difficult to 
follow. 
In all plants above the mosses, the leaf rises very near the 
apical region of the stem by a small number of cells near! the 
epidermis becoming meristematic, growing and pushing out the 
epidermis, which grows with the underlying cells so that there 
is no break between the epidermal covering of leaf and stem. 
Growth continues from the apex of the young leaf for some 
time. Afterward this apical growth ceases, the cells of the tip 
turn to lasting tissue, while intercalary growth ensues by certain 
cells near the base becoming meristematic and continuing the 
formation of new cells until the leaf is complete. From this it 
is seen that both epidermal and ground systems are included in 
the leaf. The third or vascular also forms a prominent part. 
At first the upper surface of the leaf grows more slowly than 
the under, so that its surface is concave toward the stem point, 
and forms a covering for it; later, it grows faster above and 
so straightens out. Some fern leaves grow continuously from 
the tip; and until the leaf is well grown the upper surface 
grows more slowly so that a large part is rolled completely to- 
gether, thus protecting its own tip. 
In the preceding description of the origin of the vascular 
bundles, their first appearance near the tip of the stem was 
1 In some few cases the leaf rises from the dermatogen layer itself, as in 
Elodea Canadensis. 
