Ch. VI, 2] 



STRUCTURE OP FLOWERS 



271 



In fact it would seem as though every shape that fancy can sug- 

 gest must be embodied in the shapes of the coroUas of flowers. 

 In conformity with their fleeting character, they are delicate 

 in texture, showing clearly through a lens or the microscope a 

 leaf -like anatomy of veins and 

 cortical cells, — the latter held 

 tensely expanded by the os- 

 motic pressure within. Typ- 

 ically composed of separate 

 petals (poLYPETALOus) , the 

 corolla is often, like the calyx, 

 one piece (gamopetalotjs) , 

 forming a saucer-, cup-, urn-, 

 or tube-shaped structure, 

 from the summit of which 

 spread the free petals, as 

 Primrose (Fig. 201), and 

 other garden flowers illus- 

 trate. Usually standing di- 

 rectly on the enlarged end 

 of the flower stalk, or recep- 

 tacle, the petals often stand 

 on the calyx, as in Garden 

 Nasturtium. As to mmiber, 

 petals vary like the sepals, 

 being usually, though not al- 

 waj^s, the same number as 

 they. Not infrequently the 

 petals are wanting altogether, 

 in which case the sepals often 

 replace them in color and 

 function. Sometimes the sepals are likewise absent, and 

 in this case the flower is very inconspicuous unless color 

 is supphed by bracts beneath the flower (page 74), as in 

 Poinsettia. As to the variety of colors displayed by corollas, 

 we have spoken already. 



Fig. 184. — A typical simple ovule, 

 of Polygonum divaricatum, in section ; 

 X 1.35. 



e, egg cell, in the embryo sac, 

 which lies in a mass of tissue, the 

 nucellus ; fun is the funiculus or stalk 

 by which the o\Tile is attached to the 

 ovary ; ai and ii are the integu- 

 ments, developed from the funiculus, 

 and leaving an open micropyle ; cha, 

 the chalaza. (From Strasburger.) 



