226 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



concluded that the division took place by fragmentation, but 

 Condit (1932) demonstrated the presence of mitotic figures in both 

 F. carica and F. palmata. Similarly Langdon's (1934) statement 

 that in Juglans sl period of free nuclear divisions lasting for 4 to 5 

 days is "characterized by the total absence of achromatic figures, 

 suggesting an amitotic division of the nuclei at this time" has been 

 contradicted by Nast (1935, 1941). 



The number of free nuclear divisions varies in different plants. 

 In Primula (Dahlgren, 1916), Malva (Stenar, 1925), Cochlospermum 

 (Schnarf, 1931), Brexia (Mauritzon, 1933), Mangifera (Maheshwari, 

 1934), Juglans (Nast, 1935), Malus (Wanscher, 1939), 7 Jussieua 

 (Khan, 1942), and Citrus (Bacchi, 1943) several hundred endosperm 

 nuclei may be seen lining the wall of the embryo sac. In some 

 genera, e.g., Lopezia (Tackholm, 1915), Stenosiphon (Johansen, 

 1931a), Cardiospermum (Kadry, 1946), Tropaeolum (Walker, 1947), 

 and Mclastoma (Subramanyam, 1948) wall formation does not take 

 place at all. In others like Asclepias (Frye, 1902), Rafflesia (Ernst 

 and Schmid, 1913), Leiphaimos, Cohjlanthera (Oehler, 1927), Calo- 

 tropis (Sabet, 1931), Xeranthemum (Poddubnaja-Arnoldi, 1931), 

 and Crepis (Gerassimova, 1933) it occurs at a very early stage when 

 only 8 or 16 nuclei have been formed, and in Coffca (Mendes, 1941) 

 at the 4-nucleate stage. 



When wall formation occurs, it is usually by the laying down of 

 cell plates which progress from the periphery of the sac towards the 

 center or from its apex towards the base. Less commonly, wall 

 formation may take place simultaneously in all parts of the sac as 

 in Tacca (Paetow, 1931), or it may start from the base toward the 

 apex as in Elatinc (Frisendahl, 1927), Cimicijuga (Earle, 1938), and 

 Carya (McKay, 1947). Very little is known, however, about the 

 exact origin of the partition walls. 8 In Asclepias (Frye, 1902), 

 Calotropis (Sabet, 1931), Ficus (Condit, 1932), and Gossypium 

 (Gore, 1932) minute vacuoles appear in the areas between the nu- 

 clei, and it seems that partitioning of the embryo sac takes place 

 by a process of "indentation." In some other plants the formation 

 of cell plates is preceded by the appearance of secondary spindle 

 fibers between the nuclei (Jungers, 1931). Whatever the precise 

 mode of cell formation, eventually either the entire embryo sac is 



7 Between two and three thousand nuclei have been counted in some varieties 

 of Malus. 



8 The difficulty in ascertaining the precise mechanism of wall formation is no 

 doubt due to the rapidity of the event and the poor fixation obtained at this stage. 



