724 A TEXTBOOK OF THEORETICAL BOTANY 



internal structure. The essential difference between the ovule of a Seed 

 Plant and the megasporangium of a Cryptogam is that the former is enclosed 

 by an integument. In Cycas the integument is massive and consists of a 

 stony layer with fleshy layers both outside and inside it. Although three- 

 layered it seems to be all one structure. There is a narrow micropyle with 

 a small micropylar beak. 



The megasporophyll traces come directly from the stem stele, without 

 girdles, and the vascular supply to the ovules comes from one of the collateral 



lws«,* 



s 



\ V 



jH 



A B 



Fig. 729. — Types of female cones. A, Dioon edule with loosely arranged foliar sporophylls. 

 B, Macrozmnia tniquelii with closely arranged peltate sporophylls and pendant ovules 

 shown in longitudinal section. 



mesarch bundles in the rachis of the sporophyll. This turns out into the 

 ovular pedicel and there it becomes concentric, with a solenostelic structure 

 and some centripetal xylem. This stele forks into two at the seed base, each 

 portion retaining the concentric structure and being surrounded by a distinct 

 bundle sheath and by what seems to be transfusion tissue, as well as having 

 minor accessory strands surrounding it. The further development of the 

 seed supply is show-n in Fig. 730. The vertical plane in the figure is the 

 greater diameter of the seed and is known as the principal plane. The stony 

 layer is shown by a hatched zone, and it will be seen that two branches from 

 the outer group of bundles pass through the base of the stony layer and join 

 the central bundle to form the supply bundles to the inner fleshy layer of the 

 integument, which they traverse to its apex, with slight branching. The 



