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A TEXTBOOK OF THEORETICAL BOTANY 



OVULES 



The central feature in the structure of an ovule is thenucellus, which 

 was called the nucleus in days before that name became appropriated to the 

 central feature of the cell. It is a simple, rounded or oval mass of thin- 

 walled parenchyma cells, which seldom show any differentiation. Within 

 it is developed the embryo sac, which contains the oosphere or egg-cell, 

 and around it there arise one or two coverings, called the integuments, 

 which enclose it except for a minute opening over the apex, known as the 

 micropyle. The ovule is generally attached to the placenta by a short 

 stalk called the funicle. 



The development of the nucellus on the placenta of the ovary begins with 

 the elevation of a small protuberance on the placental surface. This has 

 its origin in a certain number of sub-epidermal cells which divide repeatedly. 

 This group of active cells lies usually in the hypodermal layer and in the 

 case of certain very small ovules the origin may be traced to a single cell, 

 e.g., in Monotropa and the Orchidaceae. Exceptionally, cells of more than 

 one layer in the placenta may be involved. The placental epidermis extends 

 over the protuberance and accommodates itself to the increasing area by 

 anticlinal cell divisions. The protuberance, which is at first more or less 

 hemispherical, elongates rapidly and in doing so its basal region becomes 

 distinguishable as a supporting stalk, which will be the funicle of the ovule. 



At a very early stage, often before the appearance of the funicle, a hypo- 

 dermal cell at the apex of the nucellus shows signs of differentiation, the 

 cell and its nucleus enlarging and the cytoplasm becoming denser. This is 



Fig. 1283. — Ranunculus septentrionalis. A, Nucellus with 8-celled archesporium. B, Three 

 archesporial cells developing. C, R. abortivus. Row of four megaspore mother cells. 

 {After Coulter.) 



the archesporial cell. In some species it may even be recognizable before 

 nucellar development begins, but in any case its appearance is the first event 

 of note in that development. While the archesporial cell itself is hypodermal 

 it often appears to be the end-cell of an axile row in the nucellus (Fig. 1283). 



