ANATOMY OF THE EMBRYO AND YOUNG SEEDLING 



347 



character, that the fertihzation of synergids may be a retention from 

 multiple-egg ancestors where more than one egg was fertilized. But 

 the distribution of polyembryony scattered throughout the angiosperms 

 gives this view no support. 



ANATOMY OF THE EMBRYO AND YOUNG SEEDLING 



Little vascular tissue is mature in most embryos; procambium is 

 commonly distinct in some isolated areas, and a few mature protoxylem 

 and protophloem cells may be present. The anatomy of the embryo 

 proper is largely that of the histology of meristems. Anatomical rela- 

 tionships among the organs can be deter- 

 mined only in the young seedling where a 

 vascular skeleton is present. The vascular sys- 

 tem of the dicotyledon seedling (Fig. 133) 

 follows closely a well-known single pattern; 

 that of the monocotyledon, though basically 

 the same, varies considerably (Fig. 134). The 

 variations are related to adaptations to hy- 

 pogeal germination and are chiefly those of 

 fusions among the vascular bundles under re- 

 duction. 



Anatomy of the Cotyledon. The traces of 

 the cotyledon vary in number; two is the 

 common number in the lower taxa of both 

 mono- and dicotyledons (Fig. 7); four is fre- 

 quent, with transition to three by the fusion 

 of the middle pair; three is common with the 

 strong central bundle sometimes double (evi- 

 dence of its derivation from the four-trace 

 condition); larger numbers are probably un- 

 common — Canna, and some of the^Araceae. 

 Where there are four traces, the two median 

 traces lie close together. The Liliaceae show 

 all stages in the origin of an odd number of 

 ti-aces: two traces, four traces, a double trace, 

 a median trace with one or more pairs of lat- 

 erals. The Iridaceae have a median trace, 



simple or double, with one or two pairs of „. ,oo ci . i, r j- * 

 ^ ' t' Fig. 133. Sketch of dicoty- 



laterals; the Araceae, one to many traces. The ledonous seedling, Fijrus 



genus Scilla shows stages in the origin of the Malus. c, cotyledon; h, hy- 



median trace; the cotyledon of S. siherica has PO'^otyl; r root. {Drawing 



^.u . r c • J 1 1 by Dr. L. }. Edgerton, Cor- 



two traces, that or b. peruviana, one double nell University.) 



