THE PLANT BODY 19 



It has been argued that the concept of the monocotyledon leaf as a 

 phyllode cannot be supported by the presence of inverted bundles, 

 because similar bundles occur in petioles of dicotyledon leaves, but the 

 leaves cited — Erijngium, Ranunculus, Plantago — are also phyllodes — 

 flattened petioles, rachises, or midribs. 



As interpreted under the phyllode theory, the modification of the 

 monocotyledon leaf by reduction has gone beyond the loss of the blade. 

 The leaves of many genera of the Liliaceae and Amaryllidaceae — 

 Hyacinthus, Doryanthes, Dracaena — have petioles reduced to vestigial, 

 "solid" apices (the leafbase forms the photosynthetic structure). In 

 Hemerocallis and other taxa, even this tip is considered lost, and the 

 leafbase forms the entire leaf. 



The phyllode theory also holds that, in contrast with this reduction, 

 specialization has brought about the development of a new leaf part, a 

 secondary blade, by the expansion of the tip of the phyllode to form a 

 distal lamina. The palms, Scitamineae, Alismataceae, Smilax, Eichornia, 

 Pontederiu, are cited as examples. In such blades, parallel venation is 

 considered evidence that the "blade" is petiolar in nature, but the vena- 

 tion is commonly intermediate between parallel and reticulate. 



The theory that the simplicity of the typical monocotyledon leaf is 

 the result of reduction was discussed at the beginning of the twentieth 

 century as a part of the idea that the monocotyledons arose by the 

 "self-adaptation" of dicotyledons "to a moist or aquatic habitat." 



The blade-bearing monocotyledon leaf has been considered by some 

 to be the primitive, by others, the highly developed type. The simple, 

 ligulate leaf has been called "rudimentary," but it is associated with 

 highly specialized habit, as in the grasses. A reticulate lamina in mono- 

 cotyledon leaves may represent survival of an ancestral character or a 

 secondary structure developed as an elaboration of a part of a parallel- 

 veined leaf. The evidence that the monocotyledon leaf is, in large 

 measure, a phyllode is convincing, and there seems no doubt that the 

 "blade" of such leaves as those of Eichornia and Pontederia represents 

 the modified tip of a phyllodelike leaf. 



The interpretation of the blade of the palm leaf as an example of a 

 lamina secondarily acquired by modification of the tip of the petiole is 

 not supported by its structure or its ontogeny. Early stages of its 

 ontogeny show that this leaf is a "complete leaf" — sheath, petiole, and 

 blade. The development of petiole and blade follows the pattern of a 

 typical dicotyledon leaf; the primordium of the blade is formed on the 

 leaf buttress. This is followed by development of the petiole from an 

 intercalary meristem at its base. If the blade is to be interpreted as 

 petiolar in nature, the presence of an unusually long petiole and a 

 prominent, well-defined leafbase must be explained. 



