466 ANGIOSPERMS LttsS. 



The venter contains a single ovule or megasporangium {mg. 

 spg), differing from that of the pine in being covered by a 

 double instead of a single coat (Fig. 126, d, /^, /^), both 

 f)erforated by a micropyle (w. /j), which places the central 

 mass of tissue or nucellus (ncf) in communication with the 

 cavity of the venter (Fig. 126, a). The nucellus, like that 

 of pines, contains a single embryo-sac or megaspore (fng. sp). 



The fact that the megasporangia are contained in a cavity 

 of the carpel, and so shut off from all direct communication 

 with the exterior, forms a fundamental difference between 

 the angiospermous or covered-seeded, and the gymno- 

 spermous or naked-seeded Phanerogams. 



We saw that in Gymnosperms, as in the Vascular Crypto- 

 gams, the sporangia were borne on structures, the sporophylls, 

 which were obviously modified leaves. In the buttercup 

 the stamens and carpels have departed so widely from the 

 leaf-type that their true nature becomes obvious only after 

 comparison with other forms. 



In the White Water-lily {Nymphcea alba) the very numerous 

 petals are arranged, like the stamens, in a spiral, and the 

 two sets of organs pass insensibly into one another. As we 

 trace the petals (Fig. 125, a^) upwards on the floral recep- 

 tacle we find them become narrower in proportion to their 

 breadth (a-), and on the apex two little yellow lobes appear 

 (mi. spg). Still passing up the spiral the lobes become 

 more and more pronounced, and the petal narrower (a^), 

 until at last the lobes become aggregated into an undoubted 

 anther (a*, an), while the blade of the petal is narrowed to a 

 filament, its distal end serving to unite the anther-lobes and 

 constituting the connective {cor). 



The same transition from petals to stamens is seen in 

 many " double " flowers, such as the double apple, in which 

 the number of petals becomes greatly increased by the 



