STRUCTURE OF THE OVULE. 



433 



other cases several rows of cells are superposed, forming the 

 body known in morphology as the nucleus of the ovule. This, 

 to avoid the possibility of even slight confusion, will be now 

 spoken of as the nucellus. 



That this distinction is necessary, 

 will appear from the fact that in one 

 of the large cells of this body there 

 is a true cell-nucleus which under- 

 goes remarkable changes, all of which 

 must be described. It should there- 

 fore be remembered that in the fol- 

 lowing discussion the term nucellus 

 means exactly that which in Volume 

 I. page 277 is called nucleus of the 

 ovule. 



1125. Around the nucellus there is developed in most in- 

 stances a double ring, which soon nearly invests it, forming an 



inner and an outer coat. Attention 

 has been called in Volume I. to the 

 fact that the integuments of the ovule 

 do not complete!}- invest the nucel- 

 lus, but that there is at its true apex 

 an orifice known as the foramen or 

 micropyle. It has also been shown 

 that by a peculiar distortion during 

 its development the ovule may be 

 so bent round upon its support, the 

 funiculus, as to have the micropyle 

 present itself towards the placental 

 attachment. Hence, when the apex 

 of the ovule is spoken of, the micro- 

 pylar extremity is meant. 



1126. At the micropylar extremity of the forming ovule, a 

 single cell, beneath the surface (except in orchids and some 

 saprophytes) , elongates in the direction of the length of the ovule, 

 and by one or sometimes many transverse and vertical partitions 

 becomes divided into segments of unequal size. The lowest 

 segment continues the elongation and the enlargement of the 

 structure thus formed within the ovule, known as the embryo 



FIG. 197. Longitudinal section of the amphitropous ovule of Baptisia australia. 

 (Van Tieghem.) 



FIG. 198. Longitudinal section of the anatropous ovule of Mimosa pudica. (Van 

 Tieghem). 



