84 HOW ARE ANIMALS AND PLANTS DEPENDENT? 



stamen 



toplasm. At an early stage the pollen grain is a single cell, but at 

 the time of pollination it contains two or three cells. 



Germination of pollen grains. Pollen grains will germinate 

 if they fall on the stigma of a flower of the same kind of plant. 

 The stigma secretes a sticky fluid containing sugar and certain 

 acids, which the pollen absorbs and grows by sending out a thread- 

 like tube which causes 

 srtigmcc— ■ - ^ the growt h f the 



structure. During 

 this growth, two nuclei 

 are found in the tube. 

 One of them, the tube 

 nucleus, disappears 

 after a time. The 

 second, or germinative 

 nucleus, divides to form 

 two sperm nuclei. 



Fertilization. If we 

 cut the pistil of a large 

 flower (as a lily) length- 

 wise, we notice that 

 the style appears to be 

 composed of rather 

 spongy material in the 

 interior; the ovary is 

 hollow and is seen to 

 contain a number of 

 rounded structures 

 which appear to grow 

 out from the wall of 

 the ovary. These are 

 the ovules. The ovules, 

 under certain condi- 

 tions, become seeds. 

 The central part of the 

 style is found to be either hollow or composed of a soft tissue 

 through which the pollen tube can easily grow. In germinating, 



-Mxeceptcrcle 



Parts of a complete flower. A flower is called perfect if it 

 contains the essential organs. 



