584 



THE STRUCTURE OF ECONOMIC PLANTS 



the receptacle, below and centrad to them. These carpellary 

 primordia grow in such a way that they form three crescentic 

 structures with edges extending toward the center of the ovary, 

 and with abaxial surfaces that are adnate to the receptacle except 

 at their distal ends. The growth of their inturned edges results 

 in the formation of three ridges, each of which represents the 

 edges of two adjacent carpellary margins. As centripetal growth 

 of the ridges proceeds, they reach the center of the ovary and 



ultimately fill it except for 

 three narrow spaces which 

 form a triradiate cavity. 

 (Fig. 305, 5.) When the 

 infolded edges of the car- 

 pels reach the center of 

 the ovary, they curve out- 

 ward, grow centrifugally 

 until they approach the 

 outer wall of the locule, 

 and then again turn inward 

 toward the center of the 

 ovary. The infolded edges 

 of each carpel form the 

 placentae from which the 

 ovules arise; and in Cucur- 



FiG. 305. ^, diagrammatic longisection of young ,. /-,>r„ll„^ .„J ^^ . 



carpellate flower showing epigyny; B, transection ^Ita, ^^ItrullUS, aOQ tO a 



of ovary with three locules and multiple placentae; leSSer degree in CuCUmis, 



C diagram illustrating placenta, origin of ovules ^^^^ placenta beCOmeS a 



and their comparative development: <;/<«, corolla ; clx, tr 



calyx; w, nectary ; ovj, ovury; jf^, staminodia ; sti, multiple Structure W^lth 



stigma. (C redrawn from Kirk wood, B«//. N^w Yar^ ^^q qj. (-^j-gg longitudinal 



Bot. Card.') . , , ^, , 



ridges on each placental 

 margin. (Fig. 305, B, C.) In this manner, each carpel develops 

 a series of ovules of varying ages borne on parallel placental ridges. 

 Owing to the position in which the placental ridges are finally 

 oriented, the placentation has been described as parietal by some 

 investigators; but it is probably better to regard it as axile or 

 central placentation, plus a centrifugal overgrowth which carries 

 the placentae toward the periphery of each locule. 



The most fully developed ovules are located on the placental 

 ridge farthest from the margin of the carpel, while the smallest 

 ovules are found on the placental fold nearest the margin. As the 



