THE MEGASPORANGIUM 67 



become richly protoplasmic and the hypostase has an appearance 

 similar to that of the epithem of many hydathodes. Haberlandt 

 considers it to be a sort of glandular tissue secreting some hor- 

 mone or enzyme required for the growth of the embryo sac. 



Epistase. Van Tieghem also reported the occasional presence 

 of a similar well-marked tissue in the micropylar part of the ovule 

 and called it the epistase. Usually it originates from the apical 

 cells of the nucellar epidermis, which show a marked radial elonga- 

 tion and become somewhat thickened or suberized. Occasionally 

 the cells undergo one or more periclinal divisions to form the so- 

 called nucellar cap, which persists as a hood over the apex of the 

 embryo sac even after the cells at the sides have disorganized and 

 disappeared. 6 In Castalia (Cook, 1906) the epidermal cells lying 

 at the apex of the embryo sac show "a very pronounced sclerifica- 

 tion," and in Costus (Boehm, 1931) the inner tangential walls of 

 these cells become conspicuously thickened. In Nicolaia (Boehm, 

 1931) the walls surrounding the megaspore tetrad become cutinized 

 and form a firm covering, which becomes ruptured and separated 

 into two parts only with the continued enlargement of the embryo 

 sac. The thickenings at the micropylar end disappear but are seen 

 once again at the time of organization of the mature embryo sac. 



In some plants the apical cells of the integuments give rise to a 

 proliferation usually called the "operculum." To mention a few 

 examples, in Lemna (Caldwell, 1899) the cells forming the micro- 

 pylar portion of the two integuments enlarge and divide to form a 

 compact tissue lying just above the nucellus (Fig. 48). In Dionaea 

 (Smith, 1929) a similar tissue is formed by the cells of the inner 

 integument. In Acorus (Buell, 1935) the cells become elongated 

 and coiled around one another, so as to form a plug in the lower 

 part of the micropyle. 



Vascular Supply of Ovule. As a rule the vascular bundle entering 

 the ovule terminates at the chalaza but in some plants it gives out 

 branches, a few of which enter the integument. If two integuments 

 are present, the branches may enter only the outer integument or 

 both the outer and the inner integuments. Since integumentary 

 vascular bundles are common in gymnosperms, their presence is 

 usually considered to be a primitive feature and the loss of the con- 



6 Dahlgren (1940) designates a persistent nucellar cap of this kind by the name 

 "petasus." 



