some plants the calyx of the flower, and even the enlarged end of the stalk, 

 the receptacle, may become fused into the fleshy fruit. 



In most of the common plants the fruit will not ripen (that is, the ovary 

 will not continue its development) unless fertilization takes place. But many 

 plants ripen a seedless fruit; we have varieties of seedless oranges, seedless 

 grapes and seedless apples. The pineapple and the banana are examples of 

 fruits that develop without the ovule's being first fertilized. The plantain 

 and the breadfruit develop a more juicy fruit when the ovary is not fertilized. 

 In more recent times it has been found that ovaries of tomatoes and other 

 plants can be stimulated to grow into fruit by means of chemicals related to 

 the auxins (see page 258). 



How Does Pollen Get to the Stigma? 



Self-pollenationi In many plants the pollen is carried from the stamen 

 to the stigma by the growth movements of the parts of the flower. The style, 

 as it gets longer, may bring the stigma in contact with the anther. Or the 

 corolla, as it grows and opens, pushes the stamen against the stigma. In some 

 species the stalk of the flower may bend over as it grows, and so dumps some 

 pollen from the anther onto the stigma. In some flowers the anther stands 

 above the stigma, and the pollen is carried over by the action of gravity. Thus 

 there are many kinds of plants in which the flower may be said to pollenate 

 itself. This process is called self-pollenation and takes place in such varied 

 flowers as 



Round-leaved 

 mallow 



Sweet pea. 



Tomato 



Knotweed 



Wheat 



Obstacles to Self-pollenation There are many plants, however, in which 

 self-pollenation is quite impossible. In some species the stamens and the 

 stigmas do not ripen at the same time; self-pollenation is then impossible. 

 The pollen ripens before the stigma in maize, in the mallows, in many 

 species of the aster family, in the creeping crowfoot, and in the sage. The 

 stigmas ripen ahead of the stamens in the common plantain, in the poten- 

 tilla, or cinquefoil, and in the Oriental grass known as Job's- tears. 



^See No. 4 p. 415. 

 406 



