FLOWERS 



303 



Fertilization also seems to in- 

 duce changes in other parts of 

 the flower. The petals drop off, 

 and usually the stamens also. 

 The ovary begins to enlarge, 

 and eventually it ripens into the 

 fruit. ^ In some plants the calyx 

 of the flower, and even the re- 

 ceptacle, may become fused into 

 the fleshy fruit. 



We must be on our guard 

 against thinking of the plant as 

 an organism that looks ahead and 

 supplies the later needs of its off- 

 spring. We may say merely that 

 the plants behave in such a way 

 that the later safety and develop- 

 ment of their offspring are made 

 more probable. The baby plant is 

 protected by the mother, as well 

 as nourished, in the sense that 

 the early development takes place 

 within the ovary, and in the sense 

 that in many species hard or spiny 

 coverings are formed which prob- 

 ably prevent injury. 



1 In most of the common plants the fruit 

 will not ripen — that is, the ovarywill not 

 continue its development — unless fertili- 

 zation takes place. But there are many 

 plants in which a seedless fruit is possible. 

 Seedless oranges, seedless apples, seed- 

 less grapes, the pineapple, and the banana 

 are examples of fruits that develop without 

 the ovule being first fertilized. The plan- 

 tain and the breadfruit develop a more 

 juicy fruit when there is no fertilization. 



Fig. 134. Fertilization in a flower 



When a pollen grain,/, alights on the 

 moist surface of a stigma, s, it absorbs 

 water and puts forth a thread of proto- 

 plasm, or a pollen tube,//, which grows 

 down the style into the ovary. The tip 

 of the pollen tube finds its way to the 

 inside of the ovule, 0, through a small 

 passageway, the micropyle, m. The 

 large cell in the middle of the ovule, 

 called the embryo sac, es, undergoes a 

 number of changes which result in pro- 

 ducing several nuclei. One of these 

 nuclei at the end nearest the micropyle 

 corresponds to an egg cell. Similar 

 divisions take place in the nucleus of 

 the pollen grain, and one of the result- 

 ing nuclei corresponds to a sperm cell. 

 The cell walls separating the pollen 

 tube and the embryo sac dissolve, and 

 the pollen nucleus unites with the egg 

 nucleus. The newly formed joint nu- 

 cleus, or fertilized egg, begins to divide. 

 Thus it develops into a new plant, or 

 embryo ; the ovule becomes a seed ; 

 the ovary becomes a fruit 



