THE ANGIOSPERMAE 



1241 



hippocastanum, the Horse Chestnut, where the three-carpellary capsule 

 contains generally only one seed, although all three carpels produce ovules. 

 The familiar definition of a nut-fruit, that it is an akene by reduction, is 

 illustrated by Corylus, which has two carpels and a unilocular ovary with 

 two large placentae, only one of which bears ovules, the other remaining 

 sterile. By analogy with staminodes such 

 sterile or vestigial carpels may be called 

 carpellodes. 



The genus Carex shows the peculi- 

 arity of a sac around the gynoecium, 

 fitting more or less closely and rising 

 above the ovary to an apical opening 

 through which the long stigmas emerge. 

 This sac is called the perigynium or 

 utricle and it is a development of the 

 upper palea. Its sheathing character may 

 be compared to the basal sheaths of the 

 foliage leaves, which in this genus are 

 tubular and not split like those of the 

 Grasses. 



Among the Compositae the genus 

 Xanthium and especially X. spinosum, 

 also has sheathed gynoecia, which, how- 

 ever, arise in quite a different way. The 

 much reduced female capitulum is sur- 

 rounded by an involucral tube, covered 

 externally with hooked processes (Fig. 

 1 196). The involucre encloses two 

 naked, female flowers, which stand in 

 the axils of two modified bracts which 

 are united to form a sheath around each 

 flower. These bracts are prolonged up- 

 wards into two tubular, hooked pro- 

 cesses, very like the others but much 

 bigger, which protrude from the in- 

 volucre. The styles grow up inside these tubes and the paired stigmas 

 emerge from them through a lateral opening. The inflorescence is inter- 

 preted as a dichasial cyme, one of the two flowers being terminal and the 

 other lateral, while a third, smaller bract is usually present and is held to 

 mark the position of a suppressed, second, lateral flower. 



A syngynium, or union of two or more gynoecia belonging to separate 

 flowers, is of rare occurrence. It may be produced by the fusion of two 

 closely placed, inferior ovaries, as happens in many species of Lonicera 

 (Fig. 1197), or it may occur between superior gynoecia if the flowers are 

 unisexual, naked and closely associated, a combination of factors which does 

 not often occur. Examples are, however, available in the section Faya of the 



Fig. 1 196. — Xanthium spinosiim. Verti- 

 cal section through a female capi- 

 tulum showing the spin\' involucre 

 and the two female flowers sheathed 

 in the united bracts. {After Baillon.) 



