204 MORPHOLOGY OF THE ANGIOSPERMS 



called the stigmatic tissues. The stigma could then be loosely defined 

 as the specialized distal part of the carpel, covered wholly or in part by 

 stigmatic tissue. The stigmatic crest is a primitive form of stigma where 

 the stigmatic tissue is present along the carpel margins, below as well 

 as at the apex (Figs. 72C, 83). 



It has been claimed that, morphologically, the stylar canal represents 

 the distal parts of modified or vestigial ventral carpellary bundles, but 

 this theory is based on misinterpretation. Vestigial bundles, representing 

 lateral veins, are often present in the style and may lie beside the stylar 

 canal. Both ventral bundles and the canal are present even in one of 

 the examples cited to support the theory. (The Boraginaceae show well 

 the strands of transmitting tissue beside and distinct from the ventral 

 carpellary bundles. ) 



Placentation 



Placentation in Free Carpels. Ovules are borne on the adaxial surface 

 of the carpel lamina. Evidence from comparative studies and from 

 anatomy demonstrates that there are no cauline ovules in the angio- 

 sperms; all are borne on carpels, though some appear to be terminal on 

 the receptacle. The ovules may be distributed over most of the surface 

 of the lamina, with only a narrow marginal strip and sometimes the 

 midvein line sterile, or they may be greatly restricted in location in 

 several ways. The pattern of ovule arrangement on the carpel con- 

 stitutes placentation. The area where the ovules are attached, usually 

 more or less enlarged as an emergence of the laminar tissues, is the 

 placenta. In primitive taxa, there may be little or no modification of the 

 region where the ovules are borne; in these groups, the placenta is 

 merely a location. But position of ovule attachment is important both 

 morphologically and taxonomically. Absence of an emergent placenta is 

 probably the primitive condition; Drimijs and Dcgeneria have only slight 

 placental ridges. The ovule-bearing projection may be simple, a mere 

 low cushion or ridge, but it is frequcndy large and complex in form 

 and structure, and, together with the ovules, may nearly fill or even 

 divide the locule (Fig. 80). It may form a major part of the ripe fruit 

 and even project from the ovary walls. The form of the placenta varies 

 greatly in number of ovules, type of ovule arrangement, and form of 

 locule. When the placenta is close to the margin and where, in spe- 

 cialized carpels, the marginal band is reduced or lost, interpretation of 

 ovule position may be difficult. 



Where the fertile area is all or most of the ventral carpel surface, the 

 entire surface is the placenta. Where the fertile area is restricted to sub- 

 marginal areas and the margins are fused so that the fertile areas are 

 brought together, the placenta is double in nature, consisting of strips 



