284 MORPHOLOGY OF THE ANGIOSPERMS 



morphologically impossible, because they violate principles of under- 

 lying form; they assumed that leaves are borne on other leaves or that 

 stems are borne on leaves; somewhat similar interpretations have con- 

 tinued to the present time. 



The various theories of the nature of the ovule are in agreement 

 only in interpreting the ovule as a structure that bears the megaspores. 

 The usual interpretation today is that it is primarily a megasporangium 

 — that the nucellus is the sporangium, and the integuments, protective 

 structures. The structural relation of ovule to carpel is important but is 

 rarely mentioned. The ovule, with its integuments, funicle, and vascular 

 supply, is much more than a sorus. Among the theories of the nature of 

 the angiosperm ovule, the concept that it is an emergence of the carpel 

 lamina seems best supported, when all evidence is considered. On- 

 togenetically, it arises and develops like an emergence, an emergence 

 built up of tissues around and below an area in the primordium of the 

 lamina where fertile tissue appears early. The position of the ovules 

 on the lamina, primitively scattered, and the relation of the ovule traces 

 to the vascular system — largely or wholly derived, primitively, from 

 the minor veins of the vascular meshwork (derivation from strong, 

 lateral veins is secondary) — support the emergence interpretation. The 

 vascular supply of the ovule in origin, form, and structure is the 

 equivalent of the supply of a minor part of the lamina. It is, essentially, 

 a vein ending, with indefinite and variable branching, which is absent 

 if the tissues are few, and extensive, forming a meshwork within the 

 ovule, if they are many. The vascular system of an ovule resembles in 

 amount and branching the supply to accessory tissues in enlarged, fleshy 

 parts of fruits. 



Unlike the typical emergence, the ovule has elaborate form: protec- 

 tive lobes, the integuments; a stalk, the funicle and raphe; minor 

 adaptations undoubtedly related to enclosure in the carpel and course 

 of the pollen tube. Though there has been no general acceptance of the 

 theory that the ovule is an emergence of elaborate form, this interpreta- 

 tion was strongly supported in the last part of the nineteenth century. 

 The better understanding of type and position of the angiosperm 

 sporangium obtained in the early decades of the twentieth century 

 seems to have given even stronger support to this theory. The recogni- 

 tion that the microsporangium is wall-less and sunken in the tissues 

 of the sporophyll makes it probable that the megasporangium is similar 

 in structure and position, and gives a basis for the interpretation of 

 the ovule as an elaborate emergence. 



Enclosure of the sporogenous tissue in the nucellus and the fre- 

 quently rounded form of the nucellus have naturally suggested that 

 the nucellus is the megasporangium wall. But comparison with the 



