26o BOTANY OF THE LIVING PLANT 



by a very thin cell-wall, and is lined by dense granular protoplasm. 

 Within it seven nuceli are seen, of which one of large size is about the 

 centre (/«.). As these contents of the embryo-sac are almost constant 

 in Flowering Plants, and are all accessory to the production of the new 

 germ, they demand special attention. There are two groups of three 

 cells each, one fixed at the micropylar end, the other at the chalazal 

 end of the embryo-sac. The latter are often large, with well-marked 

 nuclei, each of which is surrounded by an area of granular cytoplasm 

 marked off by a plasmic film, not by a cell-wall. It is called the, 

 antipodal group {ant.), and it occupies the base of the embryo-sac, just 

 above the chalazal ending of the vascular strand. At the micropylar 

 end is another group of three cells, called the egg-apparatus [e.a.). One 

 of the cells projects further into the cavity than the other two : it is the 

 Ovum, or egg-cell which after fertilisation initiates the new germ. The 

 other two are of equal size, but smaller ; they are called the Synergidae. 

 The egg-apparatus is attached just below the micropyle. In Caltha 

 two layers of cells are seen to intervene (Fig. 206), but in other plants 

 the number may be larger; on the other hand in many ovules the 

 embryo-sac is found to abut directly on the micropyle. The large 

 cavity of the embryo-sac is filled by vacuolated cytoplasm, while in 

 the centre a large fusion-nucleus with a prominent nucleolus is 

 suspended by cytoplasmic threads. Though ovules of Flowering 

 Plants may vary in the form of their ovules, in the complexity of their 

 construction, in the number of the integuments, and even in the 

 number of their embryo-sacs, there is a marked constancy in the 

 number and position of the bodies contained in the embryo-sac at 

 the time of fertilisation. 



The following description of the development of the ovule relates primarily 

 to the type seen in the relatively primitive family of the Ranunculaceae. It 

 appears at first as a rounded papilla of tissue, which develops directly' into the 

 nucellus or megasporangium. By active growth and cell-division it is carried 

 up upon the elongating funiculus. Meanwhile by outgrowth of a ring of tissue 

 at the base of the nucellus the inner integument first appears. The outer 

 integument follows as a growth on the side which will be turned outwards as 

 the ovule becomes inverted ; later it coalesces with the stalk so as to invest 

 the nucellus on all sides except that of the stalk (Fig. 207). As the ovule 

 grows older the curvature increases till it is conrpletely inverted. Meanwhile 

 the integuments extend over the nucellus, covering it in, except for the 

 narrow channel of the micropyle (Fig. 206). 



The chief interest lies in the origin and developnienl of the embryo-sac. It 

 has been stated that the nucellus is a megasporangium, and the embryo-sac a 

 megaspore. It is because of the manner of their development that these parts 

 are so recognised. The young nucellus first appears as a hemispherical 



