1900] SPOROPHYLLS AND SPORANGIA OF ISOETES- 233 
and 7g are median sections of the ligule made tangentially to 
the face of the leaf. Longitudinal sections are shown in figs. 26, 
27, 28, 33, 35. Growth in length and breadth continues very 
rapid, and the ligule soon overtops the leaf (fig. 8). For 
some time it remains a single layer of cells in thickness, but 
eventually it becomes double throughout most of its extent. 
The doubling begins in the middle region near the base and 
extends in all directions, never reaching the apex or margin 
however, which remain to the last but one layer in thickness 
(fig. 21). The expanded part soon reaches its maximum 
growth. Not so the foot region; this becomes quite massive 
and deeply embedded in the tissue of the leaf, especially at the 
“sides which grow upward and downward into two prominent 
cornua. Figs. 22-25 may help to explain the form of the base of 
the ligule. Fig. 25 is a transverse section of the leaf cutting 
across the cornua above the main place of union of the ligular 
and leaf tissues. Sections below it show the cornua connected 
by a transverse band embedded in the leaf; and sections still 
lower would show portions of the cornua only. The other 
figures need no fuller explanation than that accompanying the 
plates. 
Along with the growth of the ligule there comes about a 
differentiation of the cells composing it. There may be said to 
be four regions. The base is closely surrounded by a layer of 
small deeply-staining gland-like cells (s in figs. 22,38) which 
we may call the sheath. It forms a conspicuous layer, every- 
Where investing the base of the ligule, and becoming continuous 
With the superficial cells of the leaf. Next to the sheath is an 
irregular layer or band of large empty cells, the glossopodium 
(g in jigs. 22, 38; see also figs. 23-25). The glossopodium 
appears to form the base of the ligule, but the true base includes 
the sheath which, as a study of the development shows, is 
derived from the lowermost cell of the young ligule (Jig. 38). 
Above the glossopodium are smaller cells containing protoplasm 
and forming the greater part of the ligule. The apex and 
margin of older ligules constitute the fourth region ; the cells are 
