68 



INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



ducting tissue to be an advanced one. There is, however, no defi- 

 nite evidence in favor of this view. Integumentary vascular bun- 

 dles are now known to occur in a number of families, both primitive 

 (Betulaceae, Euphorbiaceae, Ranunculaceae, and Berberidaceae) 

 and specialized (Moringaceae, Leguminosae, Punicaceae, Rhamna- 

 ceae, Convolvulaceae, Cuscutaceae, Boraginaceae, Caprifoliaceae, 

 Compositae, and Cyanastraceae). In Zizyphus (Kajale, 1944) the 

 vascular strands extend far up into the tip of the outer integument. 

 In the large succulent seeds of Hymenocallis occidentalis (Whitehead 



emb 



ABC 



Fig. 48. Development of "operculum" in Lemna minor. A, ovule showing two 

 integuments, ox and xi, nucellar cap nc, and young embryo sac. B, upper part of 

 ovule showing portion of embryo emb, nucellar cap, and two integuments; note 

 enlargement of cells of outer integument. C, embryo, nucellus, and thickened tips 

 of integuments which form the so-called "operculum." (After Caldwell, 1899.) 



and Brown, 1940) four bundles enter the ovule and during their 

 upward course they freely branch and anastomose so that a cross 

 section of a large seed shows from 14 to 18 bundles in the seed coat. 

 The inner integument of Croton (Landes, 1946) shows a network of 

 tracheids which remain conspicuous even after the other cells of the 

 integument have become flattened and crushed. 



The occurrence of vascular elements in the nucellus is much rarer. 

 Benson (1894), Frye (1902), and Benson, Sanday, and Berridge 

 (1906) identified some nucellar tracheids in Castanea, Asclepias, 

 and Carpinus respectively, but they showed no connection with the 

 vascular bundle of the funiculus. Guerin (1915) described the oc- 

 currence of connecting nucellar tracheids in some genera of the 

 Thymelaeaceae, 7 and Orr (1921a, b) reported the same in a few mem- 

 bers of the Capparidaceae and Resedaceae. 



7 According to Mauritzon (1939) all statements of the occurrence of xylem ele- 



