296 



BOTANY OF THE LIVING PLANT 



mstraction, 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 developments 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 



Fig. 217. 



Median section of a young ovule of Caltha, anatropous curvature still incomplete, 

 and the uucellus only partially covered by the integuments. The spore-mother-cell 

 has divided once, and the second division to form the tetrad is already indicated 

 by the nuclear spindles. ( x 200.) F. O. B. 



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 w T hich 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. 217). As the ovule 

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

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

 narrow channel of the micropyle (Fig. 216). 



1 he chief interest lies in the origin and development of the embryo-sac. It 



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



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



so recognised. The young nucellus first appears as a hemispherical 



