RUTACE^ 479 



Bonavia considers the rind of citrus fruits to consist of a 

 whorl of modified leaves that has grown up about the carpels. 

 The number of carpels ("sections") varies in the same 

 variety. 



Seeds. — There are from one to eight light-colored seeds 

 in each cell of the fruit. The seed coat is either leathery 

 or membranous; endosperm is lacking; each seed has two 

 or more embryos with fleshy, hypogean cotyledons. The 

 polyembryonic condition of citrus fruits is characteristic. 

 As many as thirteen seedlings from one seed have been noted. 

 Strasburger has shown that embryos of citrus seeds may be 

 derived from nucellar cells, as well as from fertilized ova. 

 He has designated such embryos as "adventitious." Hence, 

 in the polyembryonic seed, there are two sorts of embryos: 

 (i) those formed by the union of egg nucleus and sperm 

 nucleus, true sexual embryos; and (2) "adventitious" 

 embryos formed by vegetative growth. Obviously, the 

 seedlings from adventitious embryos may be used for propo- 

 gation with confidence that they will come true to the plant 

 which bore them. Early disintegration of embryo sacs 

 appears to be prevalent in citrus fruit. This may be one 

 cause, along with infertile pollen, of seedless fruits in this 

 group. 



Geographical. — Citrus species are mostly natives of the Malay Archipelago, 

 and adjacent Asiatic territory. Citrus fruits are grown only in those parts of 

 the United States where there is an almost continuous growing season, and 

 where freezes seldom occur. 



Kt.y to Principal Species of Citrus 



Petals white inside, purplish or reddish outside. 



Stamens 30 to 40; fruit 6 to 9 inches long, its skin thick, C. medica (citron). 

 Stamens 20 to 30; fruit about 3 inches "long, its skin medium thick, C. 

 limonia (lemon). 



