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 (Vaccinium), 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." 
