THE FLOWER AND FRUIT 



99 



of adjacent carpels form peripheral ridges which protrude from the 

 inner surface of the wall, but do not extend centripetally to the 

 center of the ovarian chamber. (Fig. 37, A.') 



In the axile type of placentation, the placental tissue lies at the 

 center of the ovary and may be foliar, or in part foliar and in part 

 cauline, as in the tomato. In some instances, the inturned margins 

 of the carpels extend to the center of the ovary and then are folded 

 back toward the periphery of each locule so that they appear as 

 projecting ridges extending 

 from a central axis in the 

 ovary (Cucurbita). (Fig. 



37, 5.) 



The less common type 

 is free-central placentation, 

 which occurs in the Caryo- 

 phyllaceae and Primu- 

 laceae. Dickson (x) has 

 described the placental de- 

 velopment for representa- 

 tives of each family. In 

 the former, the entire pla- 

 cental region consists of 

 the fused margins of five carpels which are separated from the 

 ovary wall by the disintegration of the intervening tissue. In 

 this manner, a form of free-central placentation is brought about 

 which is also referred to as "free-marginal" placentation. In the 

 Primulaceae, the free-central placenta is interpreted 



"as the fused margins of the carpels (plus or minus residual axial 

 tissue), which are separated from the outer portions of the carpels 

 at their origin in the receptacle, instead of breaking away after the 

 definition of the individual carpels, as in the Caryophyllaceae." 

 (Fig. 37, C.) 



Types of Ovules. — The ovule consists of a nucellus, sometimes 

 called the megasporangium, and the integuments which grow over and 

 around it. There are one or two integuments; or, infrequently, 

 there may be none, as in some of the Amaryllidaceae. 



From the placenta, a dome-shaped mass arises which develops as 

 one of the several types of ovules . In the development of the ovule, 

 the initial protuberance is formed by the division of cells of the 

 hypodermal layer which form a mass of tissue, while the over- 



FiG. 37. Types of placentation: A, parietal (dia- 

 grammatic); B, axile (diagrammatic); C, free central , 

 longisection of young flower of Primula. 



