THE PLANT BODY 



39 



cotyledons, the leaves are borne more closely than in dicotyledons, and 

 buttresses are inconspicuous or late in development, as in floral axes 

 where the organs are crowded.) Buttress and primordium may be in- 

 distinguishable. The primordium has a broader base than in the dicot- 

 yledons; the crescent margins extend far, or completely (in some 

 sheathing leaves), around the axis — an obvious relation to the sheathing 

 bases of so many monocotyledons. Marginal meristems are absent. The 

 activities of an apical meristem are brief, and the leaf is built up chiefly 



B C 



Fig. 17. Diagrammatic sketches, showing position and relationship of leaf parts in 

 primordia of the two leaf types in palms. A, B, pinnate leaf, B, showing detail of 

 furrows and ridges of pinnae primordia; C, palmate leaf; h, hook; n, rein; p, petiole; 

 r, rachis; vc, ventral crest (hastula). {From Eames, 1953.) 



by an intercalary meristem. Correlations can perhaps be found here with 

 the supposed petiolar or leafbase nature of this leaf (Chap. 12). 



Compound leaves are rare in the monocotyledons. Details of develop- 

 ment of compound monocotyledon leaves are little known. The ontogeny 

 of the compound palm leaf is described here because it is probably a 

 type unique among angiosperms. 



In the palms, the simple leaf, present in seedlings of most taxa and 

 mature plants of a few, is primitive; the compound leaf has been derived 

 from the simple. The derivation has been described as ontogenetic — by 

 a folding of the blade brought about by alternate dorsal and ventral 

 invaginations and a later splitting along the lines of the folds. The dis- 

 section is indeed ontogenetic, but the process is much more complex 

 than this. The leaf is a "complete leaf," with sheathing base, petiole, 

 and blade. The primordium is similar to that of other monocotyledonous 



