[Illustration: FIG. 245.--A carpel of _Cycas revoluta_ with 5 ovules (_s_), at half to one-third nat. size.] [Illustration: FIG. 246.--Carpel with 2 ovules of _Ceratozamia robusta_ (1/1).] =Carpels.= The simplest forms of carpels are found in _Cycas_. In this genus both the foliage and fertile leaves are pinnate, and hence present great similarity; the ovules (macrosporangia) are situated on the margin of the central portion, just as the sporangia are placed on the edge of the fertile leaf of _Ophioglossum_ (Fig. 245, compare with Fig. 209). The carpels of the other Cycadeæ present greater divergence from the foliage-leaves, being peltate, for instance, in _Zamia_ and _Ceratozamia_ (Fig. 246). The ovules in the Coniferæ are situated on the upper side and near the base of the ovuliferous scales, almost in the same position as the sporangia in the Lycopodinæ (Figs. 269, 272, 273 _H_, compare Figs. 230, 239). In _Taxus_ the uninclosed ovule is placed on the apex of a shoot (Fig. 264). In all these plants the ovules are _not enclosed_ by the carpels, that is, they are not enclosed in chambers formed by the turning in of the walls of the carpel, and hence the name _Gymnospermæ_ is given to them. In the higher Flowering-plants, the _Angiospermæ_, the ovules are distinctly situated on the edge, the upper surface, or base of the carpel; but the carpel closes round the ovules which are therefore enclosed in a chamber--the _ovary_. In a few cases, for example in the Polygonaceæ, an ovule is situated apparently on the apex of the stem itself, as in the Yew; in other cases, as in the Primulaceæ, many ovules are apparently developed on the apex of the stem, which seems to have been specially adapted as a placenta, but it is also possible and correct in these cases to suppose that the ovules are in reality developed on the carpels.[22] A single fully-developed carpel or a collection of carpels joined together is termed the _pistil_. The extremity of the carpel, which is specially developed to catch the pollen-grains and form a suitable nidus on which they may germinate, is called the _stigma_. The united edges of a carpel which bear the ovules are termed the _ventral suture_. The back of the carpel forms the _dorsal suture_. The Marsiliaceæ take a position among the Hydropterideæ analogous to that occupied by the Angiosperms; the sporangia are in a corresponding manner enveloped in a closed leaf. The collection of stamens in a flower is termed the _andrœcium_, and all the carpels, whether individually free or united into one pistil, the _gynœceum_.