CH. VII, 2] 



MORPHOLOGY OF FRUITS 



353 



The other also enters the embryo sac, and moves towards its 

 center, where it fuses with the principal nucleus of the embryo 

 sac itself (Fig. 250). This fusion nucleus, with surrounding 

 protoplasm, soon divides and forms the beginning of the 

 ^ndosperm or fonH substance later 

 used by the developing embryo. 

 ^Thua this bO called " double fertiliza- 

 tion " acts as the stimulus to endo- 

 sperm formation, and endosperm 

 formation seems clearly to act as the 

 stimulus to seed and fruit formation. 

 Incidentally this double fertilization 

 involves another important conse- 

 quence, in that the endosperm, like 

 the embryo plant, contains chromo- 

 somes from the pollen parent, of 

 which it can thus sho\y some char- 

 acteristics. A conspicuous case 

 thereof is found in Corn, where red 

 grains, the result of a red endosperm 

 showing through the grain coats, 

 appear on white ears after pollination 

 by a red variety. This phenomenon, 

 called XENIA, was very puzzling until 

 its real nature was discovered. 



While ordinarily a fruit does not 

 form, or "set" in the gardener's 

 phrase, unless fertilization has oc- 

 curred and an embryo is formed, cases 

 are known in wh'ch the fruit develops without the presence 

 of embryos in the seeds, a condition called PARTHENOCARPY. 

 In most such instances pollination is essential, as indeed 

 would be expected from the role of double fertilization ; but 

 in a few cases, notably some Figs, even pollination is not 

 essential, and fruit formation follows on flower formation 

 without any known special stimulus. 



2A 



FIG. 249. A typical 

 cone, of Fir (Abies pecti- 

 nata) ; reduced. 



Above, on right, a scale 

 with ovules ; on left the 

 seeds, with part of the scale 

 separating with them to 

 form "wings." (From 

 Sachs.) 



