1900] SPOROPHYLLS AND SPORANGIA OF ISOETES 229 
THE LEAF AND LIGULE. 
In its earliest recognizable form thé leaf rudiment seen from 
above is a crescent-shaped band of meristematic cells, curved 
about the stem apex. Sections show that it arises from the 
superficial cells of the stem apex, and is soon pushed up into a 
low broad mass, highest in the middle and inclined inwards. 
The ligule appears very early, and the leaf becomes distinguish- 
able into a proximal part somewhat triangular in section and 
destined to bear the sporangium, and a distal part approximately 
circular in section and destined to become the chlorophyllous 
region. In correlation with the rapid development of the spo- 
rangium, the growth of the leaf is at first almost confined to the 
basal region. Compare, ¢. g., the three leaves shown in fig. 8 ; 
tranverse sections would show the rapid growth of the basal 
region in a still greater degree. This region continues to widen 
as the leaf is pushed outward, by the formation of new leaves 
and the diametral enlargement of the stem; but longitudinally, 
except for a slight addition below the sporangium, there is 
only sufficient growth to accommodate the sporangium, velum, 
and ligule. 
When the sporangium is well under way the region of rapid 
multiplication and growth of cells is transferred to the part above 
the ligule. The cells here are arranged with beautiful regular- 
ity, and growth is so rapid that this soon becomes the most 
prominent part of the leaf. The maximum diameter, so far as 
the number of cells is concerned, is speedily attained, and 
growth thereafter is only in the longitudinal direction. At first 
every part of the leaf rudiment is meristematic, but in a short 
time the apex passes over into permanent tissue. This change 
into permanent tissue progresses gradually downward until 
finally the whole leaf is involved. For some time a region of 
€ver narrowing extent above the ligule continues in active 
division, but there is present no sharply marked or persistent 
meristematic zone, as seems to be implied in Farmer’s account. 
he leaf is still quite small when ail cell divisions have practi- 
cally ceased, and its further elongation, which may amount to 
