40 



MORPHOLOGY OF THE ANGIOSPERMS 



leaves. But very early, while the primordium is only 1 or 2 mm long, a 

 pattern of dissection is laid down which later determines the lines of 

 folding and splitting of the blade into "leaflets" (Fig. 17). 



The pattern of folding is laid down on both dorsal and ventral surfaces 

 of the blade primordium by a series of shallow ridges and furrows ( Fig. 

 18). Simultaneously, minute, needle-shaped openings, formed by sepa- 

 ration of cells, appear in close rows in the tissue below each ridge, dorsal 

 and ventral (Fig. ISA). These openings, at first only 10 to 20 jx in 

 diameter, do not extend to the surfaces of the blade; later, they increase 

 in length, extending to the furrows opposite them but not to the ridges 



Fig. 18. Diagrams of cross sections of parts of leaf-blade primordium of palms, 

 based on Roijstonea, showing successive stages in origin of "folds" from which 

 pinnae arise. A, early stage of differentiation, acicular slits below each ridge; B, 

 somewhat advanced stage, the slits extended to the furrows opposite them; C, later 

 stage, blade increased in thickness and in area, procambium of midveins of pinnae 

 developed, beginnings of abscission-tissue development in dorsal ridges which sep- 

 arate pinnae, d, dorsal; p, pinna; v, ventral. {From Eames, 1953.) 



(Fig. 18B). Rows of these needlelike perforations then unite, splitting 

 the ridges longitudinally from below, and forming a series of low, com- 

 pressed "folds" (Fig. ISC). Increase in tissues of the blade in area and 

 in thickness builds up the ridges (folds), which alternate on the upper 

 and lower sides and form the typical plicate structure (Fig. 19) of the 

 immature and, in many palmate types, the mature leaf. Lateral separa- 

 tion into leaflets comes about as the leaf unfolds from the bud, through 

 division by abscission layers or by "disorganization." The folding is in- 

 itiated by dissection, and the folds are built up by differential growth — 

 "invagination," in one sense. The folds are not formed by compression 

 within the sheaths of outer leaves, though, in some genera, there may 

 be a crinkling in other planes. 



This is only the first part of the story of the formation of the com- 



