Sensitive Fern 149 
The midvein of the central lobe of the trilobed blade is the 
blade’s primary midvein. As the lobe lengthens and is cut 
laterally into segments, this midvein lengthens gradually and 
becomes merged in the mature leaf’s rachis. 
The midveins of the two lateral lobes of the trilobed leaf, 
since these lobes are the incipient basal primary segments of the 
blade, are the two basal secondary midveins of the blade. In 
the mature leaf they, as well as the midveins of the primary 
segments later formed, become more or less merged in the seg- 
ments’ midribs. 
The ends of the paracostal areolz next the blade’s primary 
midvein form the bases of the midveins of the primary segments. 
The ends of the paracostal areola next the midveins of these 
segments form the bases of the midveins of the segments’ 
lobes.* Thus every paracostal areole next the rachis and 
between midribs of the primary segments reaches from midrib 
to midrib, and every paracostal areole next a midrib of a primary 
segment and between midveins of the segment’s lobes reaches 
from midvein to midvein. As the segments grow larger and 
the ends of these areola are consequently drawn farther and 
farther apart, these areole appear to be stretched while the 
rachis-wings containing them appear to shrink toward the rachis, 
until, in the base of a very large blade, they are drawn out to 
the vanishing-point and the veinlets that formed their outer 
edges are more or less coherent with the rachis, if not contained 
within it. 
The transformation of the sterile leaf into a sporophyll can 
be clearly seen from the transitional leaves which sometimes 
occur (Pl. XLV). In these, such parts of the blade’s apical 
» See page 119. 
