CHAPTER 3 



THE MEGASPORANGIUM 



The megasporangium or ovule consists of the nucellus and one or 

 two integuments. It may have various forms, which sometimes 

 intergrade into one another, and very often the same ovule changes 

 its form during the course of its development. Mature ovules are 

 usually classed under five types. In the orthotropous or atropous 

 type the micropyle lies directly in line with the hilum and above 

 it (Fig. 37 A) as in Polygonaceae, Urticaceae, Cistaceae and Pipera- 

 ceae. In the anatropous type the body of the ovule becomes com- 

 pletely inverted so that the micropyle and hilum come to lie very 

 close to each other (Fig. 375). This form is universal in almost 

 all members of the Sympetalae and is also found in several other 

 families belonging to both dicotyledons and monocotyledons. When 

 the ovule is curved, as in some of the Resedaceae and Leguminosae, 

 it is called campylotropous (Fig. 37C); when the curvature is more 

 pronounced and also affects the embryo sac, so that the latter be- 

 comes bent like a horseshoe, as in the Alismaceae, Butomaceae, 

 and Centrospermales, the ovule is called amphitropous (Fig. 37 E); 

 and when the nucellus and integuments lie more or less at right 

 angles to the funiculus as in Ranunculus, Nothoscordum, and Tulbag- 

 hia, it is called hemianatropous or hemitropous (Fig. 37 D). Ovules 

 may also be designated as epitropous, apotropous, or pleurotropous, 

 according as the inversion or bending is directed towards the top, 

 bottom, or sides of the ovary. 



A very peculiar type of ovule is seen in some members of the 

 Plumbaginaceae (Fig. 38). Here the nucellar protuberance is at 

 first in the same line as the axis, but the rapid growth on one side 

 causes it to become anatropous. The curvature does not stop but 

 continues until the ovule has turned over completely so that the 

 micropylar end again points upwards. It has been suggested that 

 this kind of ovule, also seen in Opuntia (Fig. 39), is distinctive enough 

 to merit a separate name, circinotropous (Archibald, 1939). 



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