398 PLANT MORPHOLOGY 



with megaspore formation. Consequently the egg is a diploid cell and 

 fertilization is unnecessary. Haploid parthenogenesis is very rare, having 

 been reported in only a few plants. 



The development of an embryo from other cells of the emh)ryo sac than 

 the egg, called apogamy, has been observed in Antennaria, Alchemilla, 

 Allium, Iris, and other forms, but is not known to occur constantly in 

 nature. Here a synergid or an antipodal gives rise to an embryo, which 

 may be diploid or haploid, depending on whether or not meiosis occurred 

 in the division of the megaspore mother cell. Sporophytic budding occurs 

 when cells of the nucellus or integument project into the embryo sac and 

 give rise to embryos. It has been reported in Citrus, Coelebogyne, Funkia, 

 and other angiosperms, where it frequently accompanies apogamy. 



The Fruit. The fruit develops as the seeds ripen and always encloses 

 them. Like the flower, it has no morphological individuality. A true 

 fruit consists merely of a ripened ovary, while an accessory fruit includes 

 in addition one or more associated parts of the flower, such as the calyx or 

 receptacle. The ripened ovary wall is the pericarp. When a fruit devel- 

 ops from an inferior ovary, its wall consists of the pericarp united with the 

 receptacle. At maturity, fruits may be dry or fleshy ; when dry, they may 

 be dehiscent or indehiscent. Sometimes the pericarp becomes fleshy on 

 the outside and stony within. Some fruits develop from simple pistils, 

 others from compound pistils. An aggregate fruit arises from a group of 

 separate ovaries, belonging to a single flower, that become more or less 

 consolidated. A multiple fruit is similar, except that it is derived from the 

 ovaries of a number of flowers. The development of a fruit without fer- 

 tilization is called parthenocarpy . Parthenocarpic fruits are nearly 

 always seedless. 



The Seedling. In practically all angiosperms the embryo goes into a 

 state of dormancy as the seed matures, this being accomplished by the 

 withdrawal of most of the water present and by important chemical 

 changes. In the presence of favorable external conditions, germination 

 occurs by the resumption of growth of the embryo and of other processes 

 within the seed. 



In many seeds, particularly those of dicotyledons, the endosperm is 

 completely absorbed by the embryo while the seed is ripening, the reserve 

 food being thus transferred to the embryo itself, principally to the cotyle- 

 dons. In seeds containing endosperm, this is absorbed by the embryo 

 in germination. 



When germination begins, the root tip pushes through the testa and 

 grows downward into the ground, giving rise to the primary root. The 

 hypocotyl may remain short or may elongate considerably, depending on 

 the kind of germination. Where it remains short, the cotyledon or cotyle- 

 dons remain inside the testa, the plumule soon giving rise to a shoot that 



