BOTANY OF THE LIVING PLANT 



The ovule at the period when it is ready for fertilisation is more or 

 less oval in form, and it is seated upon a stalk, the funiculus, which is 

 usually short (Fig. 2 16). It consists of a central body of conical form, 

 which is called the nucellus. This is the actual mega- sporangium. It is 

 invested by one, and frequently by two integuments, which are attached 

 to its base, and cover it closely, leaving only a very narrow channel 

 open at the apex, which is called the micropyle. The opposite end, 

 where it is attached to the funiculus, is called the chalaza. A vascular 

 strand, springing from the vascular system of the carpel, traverses 

 the funiculus, but stops at the chalazal end of the nucellus. This 

 leads up the supplies to the base of the sporangium. The form of 

 the ovule varies. Sometimes it is straight, as in the Rhubarb, or Dock 

 (Fig. 22 1, .p. 303) ; sometimes the body of the ovule is itself curved, as in 

 the Kidney bean or Shepherd's Purse. In the great majority of cases 

 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. 216, which shows an ovule of Caltha cut 

 in median section, at the time w T hen 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 for the conduction of supplies. Moreover, the nucellus 

 is the part first formed. In a young state k may be 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. 217, p. 296). 



At the period of blooming the nucellus consists of a peripheral 

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

 plants : it encloses one large cavity, w T hich, 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 by a very 

 thin cell-wall, and is lined by dense granular protoplasm. Within it seven 

 nuclei are seen, of which one of large size is near to the centre (Fig. 

 -io, /. n). 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 



