IS. 



of a pinn:;. (Fig. 3o) may correspond exuctly v/itii a seg- 

 ment wall while the upper border fallsshort of the second 

 segment wall above, or perhaps tlie reverse may "be true 

 though it was never seen satisfactorily. It is certain, 

 however, that the lower pinna on eacli side is formed from 

 tlie wliole length of one leaf segment and the larger part 

 of anotlier, and never of a single segment or of more th.an 

 tv/o. After cutting off its last seginent the apical cell 

 of the leaf, as stated ahove, probably breaks up to form 

 marginal cells like those fonned in the segments. By the 

 time that tliis lias happened, i he segments beyond the first 

 pair of pinnae, except all or part of the one next to the 

 pinna on each side, have begun to swell out (Fig. 32) to 

 form the terminal pair of pinnae. Later on the marginal 

 cells arising', from the apical cell probably take part 

 also in the development of tliese pinnae. 



The begirining of the developmen; of a pinna is best 

 seen in a. transverse section of the leaf througli the pinna, 

 and is practically the same for both pairs. I'/liile in tlie 

 segments of the petiole there are only five section walls 

 formed, and the marginal cell of the sixth grade is ■ h.e 

 ultimate one, in the segments giving rise to tlie lamina 

 no perclinal (dermatogen and plerome) walis are ever 

 formed in the marginal cells. These on tJie contrea'y con- 

 tinue to form walls paralcll to tlie lator section walls 



