THE CARPEL AND OVULE 



259 



the body of the ovule is straight, but it is inverted or anatropous, so 

 that the micropyle lies close to the attachment of the funiculus on the 

 carpel. This is seen in Fig. 206, which shows an ovule of Caltha cut 

 in median section, at the time when it is ready for fertilisation. The 

 nucellus is the essential part of the ovule, the integuments and the 

 funiculus being accessory. They provide respectively for external 

 protection, and attachment with conduction of supplies. Moreover, 

 the nucellus is the part first formed. In a young state it may be 



FIG. 207. 



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

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

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

 ( x 200.) F. O. B. 



found already well advanced, though the integuments are incomplete, 

 and the funiculus is only beginning to assume that curvature which 

 results in the inversion of the mature ovule (Fig. 207). 



At the period of blooming the nucellus consists of a peripheral 

 covering of thin-walled cells, of varying thickness in different groups 

 of plants, enclosing one large cavity, which, though its contents are 

 complex, is developed from a single cell. This is the Embryo-Sac, or 

 Megaspore. It attains its large size by encroaching on the adjoining 

 cells as it develops, by a process of digestion ; this leads to their 

 collapse, and the final absorption of their substance. The sac is limited 



