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 upon 

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

 as in pinks and primroses (fig. 258). 



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

 either singly or in clusters which occupy definite portions 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, B, an inverted, C, a bent ovule. 

 In all: /, the stalk; k, the sporangium; it. the inner integument; at, the outer in- 

 tegument ; m, the micropyle ; c, the base of the sporangium where the integuments 

 arise (called the chalaza) ; r, the ridge (rhaphe) formed by the union of stalk and outer 

 integument ; em, the megaspore. As C develops further em may become sharply bent 

 on itself.— After Prantl. 



scale-like outgrowth 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 ovules are 

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



* Although the enclosing leaves in this case do not bear the sporangia, 

 and are, therefore, not strictly sporophylls, their similarity in form renders 

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

 they are a mere roof over a convex or hollowed axis bearing the ovules. 

 (See fig. 258.) 



