CHAPTER 7 

 THE ENDOSPERM 1 



The endosperm is important because it is the main source of food 

 for the embryo. In gymnosperms it is haploid and forms a con- 

 tinuation of the female gametophyte. In angiosperms, on the other 

 hand, it is a new structure formed in most cases as the result of a 

 fusion of the two polar nuclei and one of the male gametes. Since 

 all three of the fusing nuclei are usually haploid, the endosperm 

 contains the triploid number of chromosomes (Fig. 119). la 



Endosperm formation is suppressed in two families, the Orchi- 

 daceae and Podostemonaceae. 2 In the former, triple fusion is 

 usually completed, but the fusion product either degenerates imme- 

 diately or undergoes only one or two divisions (Fig. 120). Only in 

 Calopogon (Pace, 1909), Vanilla (Swamy, 1947), and Cephalanthera 

 (Hagerup, 1947) a few free nuclei are produced, but even these soon 

 degenerate and disappear. In the Podostemonaceae there is formed 

 in the nucellus a large cavity or pseudoembryo sac (Fig. 67C,F) into 

 which the embryo is pushed down by the elongation of the sus- 

 pensor. The pseudoembryo sac seems to serve as a kind of substi- 

 tute for the endosperm, although it has an entirely different origin. 



Types of Endosperm Formation. There are three general modes 

 of endosperm formation. In the Nuclear type, the first division 

 and usually several of the following ones are unaccompanied by wall 

 formation (Fig. 121). The nuclei may either remain free or in later 

 stages they may become separated by walls. In the Cellular type, 

 the first and most of the subsequent divisions are accompanied by 

 wall formation, so that the sac becomes divided into several cham- 

 bers, some of which may contain more than one nucleus (Figs. 127, 

 132). The third or Helobial type (so called because of its frequent 

 occurrence in the order Helobiales) is intermediate between the 



l For more detailed information on this topic see Brink and Cooper (1947). 

 10 The exceptions to this condition are discussed in Chap. 13. 

 2 In Crinum latifolium also, according to Tomita (1931), embryo development is 

 often completed without endosperm formation. 



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