THE MEGASPORANGIUM 



61 



used up. 4 In the tenuinucellate forms this takes place at such an 

 early stage (even before fertilization) that some workers have mis- 

 interpreted the integument as the nucellus. Schleiden (1837) wrote 

 long ago that in the Rubiaceae the ovules are naked. Lloyd (1902) 

 demonstrated the presence of an integument in all the genera studied 

 by him excepting Houstonia. Owing to its narrow and incon- 



A 



B 



Fig. 42. Formation of nucellar beak in Polygonum persicaria. A, young nucellus, 

 showing megaspore mother cell and four wall cells; note periclinal division of a cell 

 of the nucellar epidermis. B, older stage, showing megaspore tetrad, wall cells, 

 and nucellar beak. C, mature embryo sac with part of nucellar beak; wall cells 

 have degenerated and disappeared. (After Soueges, 1919.) 



spicuous micropyle, Schleiden mistook the integument for the nucel- 

 lus, while the latter escaped his notice altogether. More recently, 

 Fagerlind (1937) has shown that even in Houstonia an integument 

 is present as usual and it is really the nucellus which is on its way 

 to extinction. He presents a series of stages to show how this con- 

 dition has been derived (Fig. 44). In Phyllis, which is at the begin- 



4 It is only in a few families like the Piperaceae and Scitamineae that the nucellus 

 persists in the seed; it is then known as the perisperm. 



