244 



PLANT LIFE. 



among gymnosperms (fig. 247), or the carpels* may form 

 a covering, as in angiosperms. In these plants the ovule 

 may terminate the axis, as in sunflower and buckwheat 

 families (fig. 257); or the ovules may be lateral iiijon 

 the surface of an enlargement of the axis within the ovulary, 

 as in pinks and prinn'oses (fig. 25S). 



It is usual, however, for the ovules to arise upon a carpel, 

 either singly or in clusters which occupy definite i.]Ortions of 

 Its surface. The cushion or ridge from which the ovules 

 arise is called the placenta. In the pines the placenta is a 



Fig. 256. — Diagrams of median longitudinal sections of three sorts of ovules to show 

 curvatures due to unsymmetric growth. A , a straight. A, an inverted, C, a bent ovule. 

 In all : /. the stalk ; /,•. the sporangium ; /?', the inner integument ; ai. the outer in- 

 tegument ; ?;/, the micropyle ; c, the base of the sporangium where the integuments 

 arise (called the chalazal ; ?-, the ridge (rhaphe) formed by the union of stalk and outer 

 integument ; tv//, the megaspore. As C develops further ent may become sharply bent 

 on itself. — After Prantl. 



scale-like outgrowtli from the upper surface of the carpel, 

 bearing two ovules ( fig. 246 ), and as the cones mature these 

 gradually outgrow the carpels and constitute the main por- 

 tion of the ripened cone. To such placentas the o\ ules are 

 attached by one side ; they are therefore entirely sessile. The 



* Althougli the cncU).sin^ leaves in thi.s ca.se do not bear tlic sporangia, 

 and are, therefore, riot strictly sporophylls, their similarity in form renders 

 it convenient to retain tlie name carpel even for those pistils in which 

 they are a mere roof over a convex or hollowed axis liearing the ovules, 

 (.See fif;, 25.S.) 



