THE CARPELS 
79 
Melastomaceae). The pollen may be smooth or warty , powdery or 
coherent, waxy , &c. ; it may be united into groups of 4 grains ( tetrads , 
Ericaceae, &c.) or masses (potlinia, Orchidaceae, Asclepiadaceae). 
Vestigial stamens or staminodes may be small and papilla-like, or 
may be petaloid as in Marantaceae, Canna, &c., or may form nectaries 
as in Loasaceae. 
The Carpels or megasporophylls of the flower, taken 
together, form its gynceceum. The megaspores and sporangia 
of Pteridophyta are represented by the embryo-sacs and 
ovules of flowering plants. The gynoeceum shows much 
variety in structure, but there is an essential homology 
running through it. 
The simple leaf-like sporophyll now only occurs in 
Selaginella and a few other forms. In the higher plants the 
megasporangia are replaced by ovules, usually borne on the 
upper edges of the carpels. The ovule contains a spore or 
spores (embryo-sacs). This germinates in situ and forms the 
female prothallus, and when the ovum is fertilised the ovule 
ripens to a seed. In Gymnospermae, as the name implies, 
the ovule (or seed) is exposed on the surface of the sporophyll, 
but in Angiospermae the sporophyll is folded inwards to form 
a hollow chamber or ovary , the ovules being borne on the 
margins (exc. Butomaceae, Nymphaeaceae, & c.) on thickened 
placentae or cushions. The tip of the carpel in the line of the 
midrib is now the style, ending in a receptive organ or stigma 
(usually sticky or hairy) for the pollen-grains. The grains, 
brought by wind or other agency, germinate on the stigma, and 
form tubes which penetrate to the ovules, carrying the male 
nuclei for fertilisation. Simple free ( apocarpous ) carpels of 
this kind are well seen in many Ranunculaceae, eg. hellebore 
or monkshood, and in Crassulaceae, Alismaceae, &c. More 
commonly the carpels are concrescent or coherent ( syncar - 
pous). In this case several types occur; there may be as 
many cavities or loculi in the ovary as there are carpels, or 
there may be only one. The former case (e.g. Myrtus, 
Fig. 7) may be imagined as carpels folded in and joined, 
the latter (eg. Corydalis, Fig. 8) as carpels meeting by their 
margins. Most commonly in the multi-locular (many- 
chambered) ovary the placentae are axile , i.e. on the inner 
walls (Figs. 5 B , 7), and in the unilocular (one-chambered) 
ovary parietal , i.e. on the outer walls (Figs. 5 A, 8), but the 
placentae may also be at the top (apical, Fig. 5 C) or bottom 
