148 Sensitive Fern 
In O. sensililis the margin of the sterile leaf-blade is very 
minutely and obscurely more or less serrulate in all stages of 
development. Aside from this, the blade is at first imperfectly 
semi-orbicular, and sometimes obscurely trilobed, with sub- 
truncate entire base and crenate or dentate upper margin. It 
then becomes successively reniform or reniform-orbicular, with 
undulate or crenate upper margin, reniform-deltoid becoming 
crenately trilobed, and deltoid trilobed with deep often squarish 
sinus at base. As development progresses, the central lobe be- 
comes sinuate or lobed, and at length cut into segments, which form 
all but the basal segments of the mature leaf and which, as they 
develop, become in their turn undulate, sinuate, or pinnatifid. 
The two lateral lobes develop into the basal segments of the 
mature leaf and become sinuous-pinnatifid. At this stage the 
segments above them range upward from sinuous-pinnatifid to un- 
dulate or entire. Development of the sterile leaf as such then 
ceases. Further change lies in its transformation into a sporophyll. 
The veins are branched and free in the first stages of the 
leaf-blade’s development. They become anastomose before 
a midvein shows in any very distinct fashion. One or two 
areola form first in the centre of the reniform or reniform- 
orbicular blade, and others quickly follow, forming outward 
until the margin has the appearance of cutting the network of 
veins. The marginal veinlets are mostly free. 
When the blade becomes trilobed the areole through the 
centre of each lobe are straightened in such a way as to form 
a double row, the inner common side of the two single rows 
that make up the double one forming the midvein. There- 
after every well-defined lobe or segment of the blade possesses 
more or less of a midvein, similarly formed. 
