FRUIT FORMATION 477 



springs are known as the placentae. Successive stages in the develop- 

 ment of a typical ovule are shown in Figs. 1 to 3 of Plate I. The ovules 

 of different species vary greatly in size, shape and degree of development 

 and differentiation. However, practically all differentiate into a central 

 portion and one or two enveloping layers. The central portion is known 

 as the nucellus, the enveloping layers as the outer and inner integuments. 

 These several structures are clearly shown in Figs. 2 and 3 of Plate I. 

 The integuments never completely enclose the nucellus but leave an 

 opening of varying size, the micropyle, through which the pollen tube 

 usually passes to effect fertilization. The stalk or filament by which the 

 ovule is attached to the ovarian wall is known as the funicle. Through 

 it the ovule and later the developing seed receives its supply of food 

 material. In many species the funicle is fused with the outer integument 

 for a short distance, giving rise to a ridge known as the raphe. The 

 point where the nucellar and integumental tissues are continuous and 

 grown together is the chalaza. 



The Embryo Sac. — At an early stage in the development of the 

 nucellus, one of its cells, the macrospore, becomes differentiated from 

 the others. This cell enlarges and divides first into two and then into 

 four cells forming the axial row. The first division of the macrospore 

 mother cell is the reduction division, which means that the number 

 of chromosomes in the nucleus of each of these four cells is half of the 

 number in the mother cell from which they were derived. Ordinarily 

 only one of these four cells develops and this becomes the embryo sac, 

 shown in Fig. 3 of Plate I. Its nucleus divides into two, then four and 

 finally eight, presenting the condition shown in Fig. 4 of Plate I. At this 

 stage the protoplasm of the embryo sac is highly vacuolated. At one 

 end, three of the nuclei are visible, constituting the egg apparatus. Only 

 one is capable of being fertilized. The other two are called synergids; 

 their exact function is not known. At the opposite end of the embryo sac 

 are three nuclei called antipodals which are separated at an early stage 

 from the rest of the sac contents by the formation of cell walls. These 

 cells do not take any direct part in the process of fertihzation and they 

 do not influence the development of the fruit so far as known. Sooner 

 or later they, like the synergids, disintegrate. Near the center of the 

 embryo sac are the other two nuclei called polar bodies because each has 

 come from the group of nuclei at the extreme ends or poles of the embryo 

 sac. These nuclei fuse and divide rapidly, forming the endosperm; in 

 many instances one of the male gametes unites with the fusion nucleus 

 bringing about double fertilization. 



Pollen. — Stamens originate as small protuberances at their points 

 of insertion on the axis of the flower. At first these projections consist 

 of homogeneous tissue, but differentiation soon occurs and it becomes 

 possible to recognize filament and anther. The anther increases in 



