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 areolae next the blade's primary 

 midvein form the bases of the midveins of the primary segments. 

 The ends of the paracostal areolae 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 areolae are consequently drawn farther and 

 farther apart, these areolae 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 (PI. XLV). In these, such parts of the blade's apical 



* See page 119. 



