208 MORPHOLOGY OF THE ANGIOSPERMS 



placentation, is apparently rare — Helobiales, Nymphaeaceae. Basal and 

 suspended placentation in the syncarpous ovary may have a more 

 complex origin but represent derivation indirectly from submarginal. 



It has been urged that the term parietal be applied to placentation 

 in free carpels of the follicular type, here called submarginal, but 

 parietal has long standing as applied to a type of placentation in syn- 

 carpous ovaries and, so used, is a good descriptive term. Parietal, in its 

 meaning of "on the wall," is applicable to both submarginal and laminar 

 placentation, but seems best restricted to syncarpous ovaries, where it 

 conti-asts well with placental positions of markedly different morpho- 

 logical types. It has also been used as a synonym for laminar, especially 

 where the ovules are very few and are isolated on the carpel wall be- 

 tween the median and venti'al veins. 



Though "marginal" has long been generally accepted as a simple and 

 primitive ovule position, the term submarginal — now commonly used — 

 was applied to this type of placentation as early as 1850, but it did not 

 continue in use. "Marginal" is especially undesirable, because it may 

 suggest a nonlaminai" position, a position on the edge of the carpel. 

 The critical study of the highly primitive carpels of the woody Ranales, 

 together with ontogenetic studies in many taxa, has demonstrated that 

 the primitive position of the so-called marginal ovules is submarginal, 

 and "marginal" is incorrect, morphologically (Fig. 83). Study of the 

 placentation of free carpels in many families shows that the submarginal 

 position is characteristic of most taxa. In some taxa, die ovules are 

 apparently borne on the edge of the lamina, but these are specialized 

 carpels with lamina reduced and the strip of blade between the ances- 

 tral ovule position and the margin narrowed or lost. This reduction is 

 frequent in syncarpous ovaries (Fig. 84). These "marginal" ovules are 

 shown by position of their primordia to be submarginal. 



Laminar placentation is probably present only in families generally 

 accepted as primitive — Nymphaeaceae, Cabombaceae, Butomaceae, 

 probably all of the Helobiales, Lardizabalaceae. There is strong evi- 

 dence that laminar placentation is the ancestral type. In this type, the 

 ovules, typically, are distributed over the entire lamina, as in Butomus, 

 Hydrocharis, Nijmphaca, and derive their vascular supply chiefly from 

 the smaller meshwork bundles of the lamina, rarely directly from the 

 median or major lateral veins. In reduction in ovule number and re- 

 striction to the submarginal position, a marginal sti'ip of the lamina and 

 the median line become sterile, and the ovules derive their vascular 

 supply from two major lateral bundles, one on each side, not from the 

 laminar vascular meshwork. The Degeneriaceae and Winteraceae show 

 evidence of transition from laminar to submarginal placentation. In 

 Degeneria, though most of the ovules derive their vascular supply 



