THE PISTIL 463 



Structure of the Angiosperms, had to be mtroduced. That the 

 ovary gives the Angiosperms a special economic importance is 

 attested by the fact that our fruits are either ripened ovaries or 

 ripened ovaries plus closely related parts. Within the ovary 

 occur the cavities or locules in which are borne the megasporangia 

 or ovules, varying in number and also in the way they are at- 

 tached in different Angiosperms. 



The ovule is generally borne on a stalk (funiculus), and the 

 chief structure of the ovule is the nucellus, which in most Angio- 

 sperms is enclosed by two integuments, an inner and an outer one. 

 As in Gymnosperms, the integuments do not completely close 

 over the top of the nucellus, but leave a small opening (micropyle). 



Usually the ovule curves as it develops and the micropyle is 

 brought around to near the base of the ovule. This position of 

 the micropyle is a favorable one for the entrance of the pollen 

 tube. There are terms used to indicate the amount of curving 

 ovules undergo in their development. Ovules that remain 

 straight are orthotropous. Those that double clear back upon 

 themselves are anatropous. Those turning only part way back 

 upon themselves are campylotropous. Within the nucellus is 

 formed the megaspore mother cell (B, Fig. 408), which also 

 divides by two successive divisions in one of which the number of 

 chromosomes is reduced to the gametophytic number. A mega- 

 spore. therefore, produces four megaspores, each of which is com- 

 parable to a pollen grain. Although the megaspores are formed 

 while the flowers are mere buds, they are formed later than the 

 pollen grains. As in the Gymnosperms, in most Angiosperms 

 only one of the megaspores develops into a gametophyte, although 

 among Monocotyledons, there are cases in which more than one 

 or all of the megaspores apparently take part in forming the one 

 gametophyte. The lower diagram in Figure 409 gives the usual 

 history of the megaspores. 



The female gametophyte is very much reduced, consisting of 

 only a few nuclei and naked cells in a small mass of cytoplasm. 

 In most Angiosperms the female gametophyte is developed in 

 the following way. The megaspore first enlarges by digesting 

 and using the other three megaspores and the adjoining cells of 

 the nucellus as food. Then as the megaspore further enlarges 

 the nucleus divides, and the daughter nuclei pass to opposite 

 ends of the embryo sac which is the term now applied to the 



