THE OVULE 273 



morphologically significant from the standpoint of the evolution of 

 placentation. In laminar placentation, ovule traces are derived from all 

 parts of tlie vein system; few or none are derived from the midvein in 

 some taxa. In submarginal placentation, the traces are from the ventral 

 veins. Most solitary, suspended, or basal ovules also have traces from 

 ventral bundles, though the morphological origin may be obscure. Ex- 

 amples are seen in the "basal" ovules of some achenes, where the trace 

 seems to be derived from the midvein — lateral and midveins are re- 

 duced and fused (Fig. 42K, L, M) — and in syncarpous ovaries, where 

 the terminal ovule is a surviving member from a free central placenta 

 and its trace is derived from the fused ventrals of all the carpels — 

 Cyperaceae, Polygonum (where the ovule has six traces or one com- 

 pounded of six). Rarely, in two-trace ovules, one of the traces has 

 similarly been "taken over" functionally by a nearby ovule — Ilex, Uiiica 

 gracilis. Ilex verticillata has two basal ovules, one abortive, each with 

 one trace; I. crenata has one basal ovule, with two traces derived from 

 the ventral traces of different carpels. In the Cornaceae, the ovules of 

 Cornus and Corokia have been shown to have more than one trace. In 

 Cornus, the ovules of some species have two traces, one from each 

 ventral bundle of one carpel, and other species have several traces — 

 the extra traces apparently "taken over" from lost ovules. (Ovule traces 

 are sometimes confused with carpellary tiaces, with resultant misin- 

 terpretations of carpel structure. Ovule traces are branches of the vas- 

 cular system of the carpel and not derived from the receptacular stele. 

 Under great reduction in the carpel, the ovule trace may be the distal 

 continuation of the carpel trace.) 



The presence of vascular bundles in the integuments of amentiferous 

 genera — long considered some of the most primitive dicotyledons — and 

 in the primitive gymnosperms was considered, at the beginning of the 

 twentieth century, evidence that ovules with vascularized integuments 

 are primitive. That vascular bundles occur in many other taxa, includ- 

 ing such gamopetalous families as the Cucurbitaceae, Oleaceae, 

 Caprifoliaceae, and Compositae, seems, surprisingly, to have been over- 

 looked. Ovule integuments with vascular bundles have been described 

 in more than one hundred species in over thirty families, scattered 

 throughout the angiosperm system (Figs. 104 and 105). Their distribu- 

 tion is erratic; they are present in many genera of some large families 

 such as the Compositae, Leguminosae, Euphorbiaceae; they may be 

 absent in some species of a genus, though present in others; no vascu- 

 lar tissue has been reported in the integuments of the Primulales, 

 Rhoeadales, Ericales, Rhamnales, Proteaceae, Polygonaceae. In families 

 that generally lack them, isolated genera may have them — -in the 

 Ranunculaceae, only Anemone and Glaucidium, and in the Rosaceae, 

 only Prunus are known to have them. 



