120 



THE PISTILS. 



[LESSON 18. 



318. With a Free Central Placenta, is what *r< Snd in Purslane 



(Fig. 214), and in most Chickweeds (Fig. 258, 259) and Pinks. 

 The difference between this and the foregoing case is only that the 

 delicate partitions have very early vanished ; and traces of them 

 may often be detected. Or sometimes this is a variation 

 of the mode 



319. With Parietal Placenta?, namely, with the ovules 

 and seeds borne on the sides or wall (parietes) of the 

 ovary. The pistil of the Prickly Poppy, Bloodroot, 

 Violet, Frost-weed (Fig. 261), Gooseberry, and of 

 many Hypericums, are of this sort. To understand it 

 perfectly, we have only to imagine two, three, or any 

 number of carpel-leaves (like that of Fig. 

 251), arranged in a circle, to unite by their 

 contiguous edges, and so form one ovary 

 or pod (as we have endeavored to show in Fig. 260) ; 

 very much as in the Stramonium (Fig. 199) the 

 five petals unite by their edges to compose a mono- 

 petalous corolla, and the five sepals to form a tubular 

 calyx. Here each carpel is an open leaf, or partly 

 open, bearing ovules along its margins ; and each 

 placenta consists of the contiguous margins of two 

 pistil-leaves grown together. 



320. All degrees occur between this and the sev- 

 eral-celled ovary with the placentce in the axis. Com- 

 pare, for illustration, the common St. John's-worts, Fig. 255 and 256, 

 with Fig. 262, a cross-section of the ovary of a different species, in 

 which the three large placentae meet in the axis, but 

 scarcely unite, and with Fig. 263, a similar section of 

 the ripe pod of the same plant, showing three parietal 

 placentas borne on imperfect partitions projecting a 

 little way into the general cell. Fig. 261 is the same 

 in plan, but with hardly any trace of partitions ; that 

 is, the united edges of the leaves only slightly project into the cell. 



FIG. 258. Pistil of a Sandwort, with the ovary divided lengthwise ; and 259, the same 

 divided transversely, to show the free central placenta. 



FIG. 260. Plan of a one-celled ovary of three carpel-leaves, with parietal placentae, cut 

 across below, where it is complete ; the upper part showing the top of the three leaves it is 

 composed of, approaching, but not united. 



FIG. 2CL Cross-section of the ovary of Frost-weed (Heliantheimun), with three parietal 

 Vlacentse,, bearing ovules. 



