OVULES OF CYCADS 



325 



siderable size, but contain only a single large megaspore (p.) 

 within the several-layered parenchymatous wall or nucellus (n.). 

 The delicate texture of the latter can be related to the presence 

 of a thick fleshy protective covering (o. and i.). This integument 



FIG. 1 86. A, Megasporophyll of Cycas revoluta with four ovules (about 

 half natural size). B, Mature megasporophyll of Encephalartos hil- 

 denbrandtii, bearing two seeds (about half natural size). D, Diagram 

 of longitudinal section of ovule of Bowenia spectabilis, and C, front end 

 of same enlarged, ar., archegonia ; i., inner fleshy layer of integu- 

 ment ; i.b., vascular bundle of inner series ; mi., micropyle ; n., 

 nucellus ; o., outer fleshy layer of integument ; o.b., bundle of outer 

 series ; p., female prothallus within megaspore ; p.c., pollen chamber ; 

 p.t., pollen tube ; s., stony layer of integument. (A and B original ; 

 C and D after Kershaw.) 



completely envelops the nucellus, and is indeed fused with it 

 except at its extreme apex. In this region the integument is 

 pierced by a narrow canal, the micropyle (mi.), leading down to the 

 tip of the nucellus, in which a conical cavity, the pollen chamber 

 (p.c.), is formed by the breaking down of the tissue. The ovule is 



