18 MORPHOLOGY OF THE ANGIOSPERMS 



tary carpel or stamen and a basal ovule (in a syncarpous ovary) may 

 acquire a terminal position in a floral shoot. The "terminal" cotyledon 

 of many monocotyledon embryos is also secondary (Chap. 9). Change 

 of position of these organs, congenitally established, does not make 

 such a leaf, carpel, ovule, or cotyledon morphologically cauline, as has 

 been sometimes claimed. The error of interpretation of basal ovules as 

 cauline because of their apparently terminal position has brought about 

 suggested changes in the classification of major angiosperm taxa that 

 are morphologically unsound: the division of angiosperms into Phyllo- 

 sporeae and Stachyosporeae, and the removal of the Casuarinaceae 

 from the angiosperms. 



The monocotyledon leaf is characteristically simple; compound forms 

 are rare — Dioscoreaceae, some of the Liliaceae and Araceae. Parallel 

 venation is dominant; the compound leaves and some simple leaves 

 have parallel-reticulate, palmate-reticulate, or pinnate-reticulate vena- 

 tion — Trillium, Smilax, Colocasia, Arisaema, Butomaceae, Pontederiaceae, 

 Alismataceae. The simple leaf is elongate, commonly linear, and con- 

 sists of a sheathing base — the leaf sheath or leafbase — and a distal 

 "limb"; these parts may merge, or be more or less clearly delimited. 



The resemblance of the simple, linear monocotyledon leaf to the 

 phyllodes of some dicotyledons suggested the plu/Uode theory, the 

 conception that this leaf represents morphologically the basal sheath or 

 petiole of an ancestral leaf. The sheathing base is looked upon as an 

 adaptation in the development of the characteristic habit in mono- 

 cotyledons — leaves close-packed on a greatly shortened stem. In the 

 parallel venation of the "limb" is seen the vascular pattern of a petiole. 



Morphological support for this theory is supposedly given by the 

 close resemblance of the monocotyledon leaf to the sheathing bases of 

 some dicotyledon leaves — Umbelliferae — and of the resemblance of the 

 terete "limb" to petioles in such genera as Triglochin and species of 

 Allium, Sagittaria, Sisyrinchium. Vascular anatomy is considered to 

 support the interpretation of the monocotyledonous leaf as homologous 

 with the proximal part of the dicotyledonous leaf. Parallel venation is 

 characteristic of sheathing leafbases and petioles. Species of Sagittaria 

 are believed to show stages in the flattening of terete leaves. The 

 flattened leaves show two series, dorsal and ventral, of vascular bundles 

 oriented with phloem toward the dorsal and ventral surfaces, respec- 

 tively. Bifacial leaves, so formed, show stages in the merging of these 

 series, with, in the completely flattened leaves of some taxa, all the 

 vascular bundles in one plane and alternating irregularly in orientation. 

 This vascular structure is present also in the leaves of the Ponte- 

 deriaceae, Hydrocharitaceae, and other families. An apparently dorsi- 

 ventral leaf is shown by its anatomy to be a modified cylindrical leaf. 



