38 



MORPHOLOGY OF THE ANGIOSPERMS 



A-- 



it becomes active. The petiolar meristem may remain inactive for a 

 long period, or may continue cell division, with little cell enlargement. 

 The petiole may develop either from a transverse meristematic plate, or, 

 more commonly, by the rapid enlargement of small cells. 



The midrib region of the leaf is formed by the central part of the 

 early elongate primordium, the remainder of the blade by specializa- 

 tion of the tissues formed by the marginal meristems. The history of 



the various tissue layers of the 

 blade is complex but follows a 

 general pattern. 



The morphological nature of the 

 basal shoulder of the primordium 

 — leaf buttress — on which the leaf 

 arises is discussed earlier in this 

 chapter. Ontogeny shows that it 

 exists as a topographical feature of 

 the shoot development, that it is a 

 part of the shoot meristem not 

 referable to the mature leaf or 

 axis. In the mature shoot, it is a 

 region where leaf and stem merge. 

 Fig. 16. Transverse sections of edges of Early stages in the development 

 young laminae in progressive stages of r ^ i ^^ iiir 



development, showing subepidermal cell ^^ ^^e compound dicotyledon leaf 

 divisions that bring about marginal are the same as those of the simple 

 growth in the young lamina A, initial leaf. Evidence that the leaf will be 

 cell; B, daughter cell of A by vertical -, . i r. i-i 



division; A\ A\ daughter cells of A by compound appears m the fingerlike 

 horizontal division; B\ B', daughter cells stage of the primordium, with the 

 of B by vertical division; u ep, upper development of leaflet primordia 

 epidermis; ti m, upper mesophyll; m m, ■, \, • i . i r. 



middle mesophyll; I m, lower mesophyll; ^^^^g ^^^^ marginal ridges. Se- 

 l ep, lower epidermis. {After Avery.) quence in development in the 



leaflet primordia varies with the 

 taxon— acropetal, basipetal, or "divergent." Apparently little is known 

 of the origin of these new apical and marginal meristems from the 

 margins of the mother primordium— whether they appear before or 

 after the mother marginal meristems are established. Leaflet develop- 

 ment follows the same course as that in the simple leaf. Ontogenetic 

 fusion between leaflet primordia apparently may occur, as between 

 floral-organ primordia, but is infrequent or rare and little is known 

 about it. (Leaflet fusion in Bauhmia and related genera is apparently 

 largely congenital. ) 



The history of development of the leaf of the monocotyledons differs 

 only in detail from that of the dicotyledons. The leaf buttress is less 

 prominent than in dicotyledons and may be absent. (In many mono- 



