POLLINATION AND FERTILIZATION 159 



green-colored member known as the calyx, divided into 

 several leaf-like parts, each of which is called a sepal. The 

 calyx protects the young flower-bud from insects, winds, 

 rain, and possibly in some cases from cold. Inside of the 

 calyx is the colored showy part of the flower known as the 

 corolla, the divisions of which are called petals. The bright 

 colors of the corolla attract bees and other insects. A 

 further examination of the flower reveals a number of slender 

 members, the stamens, or male organs of the plants, each 

 consisting of a thread-like part (the filament) and a knob- 

 like enlargement (the anther). Pollen-grains are produced 

 within the anthers. The pistil or female part of the cherry 

 blossom is a vase-shaped organ in the center of the flower. 

 The somewhat expanded portion of the top, known as the 

 stigma, receives and holds the pollen-grain at the opportune 

 time and secretes substances to cause the grain to ger- 

 minate. At the bottom of the pistil is a bulge, the ovary or 

 seed case, which, after fertilization has taken place, holds the 

 developing seed. 



257. Pollination and fertilization. Before a seed can be 

 formed, a certain cell contained in the pollen-grain must be 

 brought in contact with a particular cell in the ovary so that 

 the two cells may unite or fuse. When the pollen-grains are 

 mature, the anthers crack open (dehisce) and liberate them. 

 They then fall upon or are carried to the stigma. The 

 alighting or placing of the pollen-grain upon the stigma is 

 known as pollination. 



Soon after the pollen-grain has alighted upon the receptive 

 stigma, it begins to swell and puts forth a tube. The pollen- 

 tube continues to lengthen, grows down the style, and 

 finally reaches the ovary through a little opening called the 

 micropyle. The nucleus is then discharged from the tip of 

 the pollen-tube, and fuses with the female nucleus in the 

 ovary. This process is called fertilization (Fig. 100). After 

 the two cells have fused, a division takes place (Fig. 101). 



