Leaf and Sporocarp in Marsilia quadrifolia, L, 125 
Stomata also occur on the petiole, but apparently not until 
quite a late stage, and their development was not studied. 
When the longitudinal partitions are about three or four 
cells in width (radially), longitudinal rows of mesophyll-cells, 
usually one opposite each partition, have become specialized 
to form the so-called tannin-sacs (t. Figs. 9, 10). The cells 
composing these are, like the surrounding mesophyll-cells, 
about twice as long as broad at maturity and rounded off 
laterally, forming many small intercellular spaces connecting 
with the large air-canals. 
While the dermatogen and periblem have been developing 
as described above, the plerome of sections I-IV has given 
rise to the axial vascular bundle of the petiole. The plerome 
of section II divides by two longitudinal anticlines into 
quarters, of which the one in the angle between I and III 
never divides further in any direction, but forms the large 
trachea of its side of the bundle (tr., Figs. 3, 8-10, 22). The 
nucleus of this cell may divide many times, so that in a 
trachea of half a millimeter in length (Fig. 3) we may find 
twenty-five or thirty nuclei, but these and all other proto- 
plasmic contents disappear later and the end-walls assume 
the characteristic oblique position, always with the dorsal 
edge directed towards the base of the leaf. 
The remaining three quarters of this section and all the 
plerome of sections I, III, and IV break up by numerous 
longitudinal walls (Figs. 8-10), and later by fewer transverse 
walls (Fig. 3), to form the remaining tissues of the bundle 
which later still develops a bundle-sheath (b. s., Figs. 9, 10). 
The portion of the marginal cell within what we have called 
the plerome-wall never forms any part of the vascular bundle, 
and the same is usually true of the same portion of section V, 
though it does occasionally form a small portion of it (Fig. 9). 
The Lamina. 
Just before the activity of the apical cell ceases, the tenth 
and eleventh (or eleventh and twelfth) segments on each side 
begin to grow out laterally and ventrally to form the first 
