THE ANGIOSPERMAE 1225 



its margins united adaxially (or ventrally) and it is common to find a single 

 row of ovules on each of these margins. Much less commonly one or more 

 rows of ovules may lie along the dorsal side, attached to what would be 

 regarded as the mid-rib of the carpellary " leaf ". Two other modes of 

 attachment are to be found in single carpels, namely basal placentation, 

 where one or, more seldom, several ovules are attached to the bottom of the 

 loculus, or pendulous placentation, where the ovule or ovules appear to hang 

 from the top of the loculus. A fifth method of attachment is also possible, 

 in which the placenta appears to cover the whole inner surface of the 

 carpel, and ovules are distributed all over it, or, when only a few are formed, 

 are scattered irregularly. These five modes of placentation comprise 

 between them all the possibilities and they are all to be found in single 

 carpels. The very various placentations found in compound ovaries are all 

 derivable from them, or in other words, the carpel as a unit of a compound 

 ovary does not differ from a free carpel in its modes of ovulation, except in 

 one respect, that where carpel margins have become fused to those of 

 neighbouring carpels in a compound ovary, the number of ovules which 

 they bear may be greatly increased, perhaps as a result of more effective 

 nutrition. 



Basal placentation has always attracted theorizers because ovules in this 

 position, particularly where there is a single carpel, often appear to be 

 direct upgrowths from the floral axis and thus in a different category from 

 those which are borne on the carpels. At a time when ovules were held to be 

 the equivalent of buds it was not permissible to regard them as borne on 

 carpellary " leaves ", because it was a dogma of the older morphology that 

 leaves cannot produce buds. Therefore, it was argued, all ovules must be of 

 axial origin and, beginning with the simple and apparently obvious case of 

 the basal ovule, placentae of axial nature were supposed to have grown 

 up and attached themselves variously to the carpels. When eventually it 

 was demonstrated that ovules were not buds but sporangia, opinion swung 

 in the other direction; the carpels were interpreted as sporophylls and the 

 attempt was made to show a foliar origin for all ovules. Under this inter- 

 pretation the basal ovule was an outstanding difiiculty. Celakovsky 

 endeavoured to get over it by comparing the carpel to an ascidial leaf, 

 growing from a hypothetical Sohle, a sort of basal plinth which, although 

 united to the floral axis, was really a part of the carpel itself. From this 

 grew the idea that carpels could be compared to peltate leaves, which, as 

 we have seen above (p. 1215), does in fact appear to hold good in many 

 cases, though not in all. Goebel and Troll have both supported this view and 

 the former retains the term Sohle for the adaxial lip of the peltate carpel, 

 which is often so little developed, in comparison with the dorsum, that it 

 scarcely rises above the base. Ovules (usually single) borne on it may 

 appear, in the mature carpel, to be basal, the dorsal portion of the carpel 

 being sterile and its margins only infolded above the level of the adaxial 

 Sohle or " sill " as we have translated it, following Goebel's view (see 

 footnote, p. 1 2 14). Troll, indeed, has pointed out that the facts of develop- 



