SEED-BEARING PLANTS 451 



401. Essentials of a Fruit. In the Gymnosperms we 

 found the seeds unprotected on the surface of the mega- 

 sporophyll or carpel; but in the Angiosperms the ovules 

 are produced in a closed ovary composed of one or more 

 carpels (Fig. 334). As the ovules ripen into seeds the 

 carpels and surrounding parts ripen into the fruit. In 

 some cases the fruit consists only of the ripened ovary 

 (Fig. 334) while in other cases it may comprise the enlarged 

 calyx and receptacle also (Fig. 335). 



402. Immediate Effect of Pollen. The effect of the 

 germinating pollen in stimulating the growth of the ovary 

 and adjacent tissues is a very interesting phenomenon. 

 A portion of the edible part of the fruit of apples is calyx, 

 which has developed into fleshy tissue as a result of 

 the stimulus of the pollen; in the case of pears the re- 

 ceptacle and end of the peduncle become fleshy and form 

 a part of the fruit; most of the strawberry fruit is the 

 common receptacle of the small flowers, stimulated 

 to a fleshy development by the growth of the pollen 

 on the stigmas; in the watermelon, orange, tomato, and 

 many other plants, it is the ovary alone that is thus 

 stimulated. 



The immediate effect of pollen is often greatly increased 

 by cross-pollination. This is strikingly shown in the 

 blueberries (V actinium), as shown in Fig. 336. The 

 two twigs "grew in equally good situations on the same 

 bush, contained the same number of flowers, all pol- 

 linated by hand with equal care, and the fruits were pol- 

 linated on the same day. The only difference in treat- 

 ment was that the pollen used on the left-hand twig came 

 from other flowers on the same bush, while the pollen for 

 the right-hand twig was taken from another bush." 



