678 



ACANTHACE^. 



[Perigynous Exogens. 



Order CCLXIII. ACANTHACEiE.— Acanthads. 



Acanthi, Jiiss. Gen. 102. (1789).— Acanthacea?, R. Br. Prodr. 472 ; Nees ah. Esenb. in Wall, 

 pi. as. rar. 3. 70 ; Endl, Gen. cl. ; Meigner Gen. p. 293. 



Diagnosis. — Bignonial Exogens, with axile placentce, wingless exalbuminous seeds attached 

 to hard placental processes, and large fleshy cotyledons. 



Herbaceous plants or shrubs, chiefly tropical ; their haii's, if they have any, simple, 

 occasionally capitate, A-ery rarely stellate. Leaves opposite, rarely in fours, without 



stipules, simple, undixdded, entire, or ser- 

 rated ; rarely sinuated, or having a ten- 

 dency to become lobed, sometimes in 

 unequal pairs. Inflorescence termmal, 

 or axillary, in spikes, racemes, fascicles, 

 or panicles ; the flowers sometimes even 

 sohtary. Flowers usually opposite in the 

 spikes, sometimes alternate, with three 

 bracts, of which the lateral are now and 

 then deficient ; these bracts sometimes 

 large and leafy, and inclosing a dimi- 

 nished calyx, which is occasionally obso- 

 lete. Calyx 4- or 5-divided, usually 5- 

 leaved, equal or imequal, generally very 

 much imbricated, occasionally cut into 

 many pieces, or entire and obsolete, per- 

 sistent. Corolla monopetalous, hypogy- 

 nous, bearing the stamens, mostly irre- 

 gular ; the limb ringent or 2-lipped (the 

 lower lip overlapping the upper in aesti- 

 vation), occasionally 1 -lipped, sometimes 

 nearly equal, deciduous. Stamens mostly 

 2, both bearing anthers ; sometimes 4, 

 didynamous, the shorter ones being some- 

 times sterile ; anthers either 2-celled, 

 their cells being inserted equally or unequally, or 1 -celled, opening lengthwise. Ovary 

 seated in a disk, 2-celled, composed of 2 carpels placed in front and back as regards the 

 axis, and bearing the placentae on then- edges, the cells either 2- or many-seeded ; pla- 

 centse parietal, although adhering in the axis ; style 1 ; stigma 2-lobed, rarely undivided ; 

 ovules amphitropal or campulitropal. Capsule 2-celled, the cells 2- or many-seeded, 

 often contracted into a stalk by the abortion of the base, and sometimes even 1 -seeded, 

 bursting elastically with 2 valves. Dissepiment opposite the valves, separating into two 

 pieces through the axis (the middle being sometimes open) ; these pieces attached to the 

 valves, sometimes separating from them with elasticity; entire, or occasionally sponta- 

 neously splitting in two, their inner edge bearing the seeds. Seeds roundish, hanging 

 by hard, cup-shaped, or usually hooked ascending processes of the placenta ; testa 

 loose ; albumen ; embryo curved or sti'aight ; cotyledons large, roundish ; radicle 

 taper, descending, and at the same time centripetal, cm-ved, or straight. 



In a majority of cases these plants are to be recognised by the presence of 

 large leafy bracts, in the axils of which the flowers are concealed, and also by their 

 calyx being composed of deeply imbricated sepals forming quite a broken whorl. But 

 their most exact difference from the other Orders of the Alliance consists in the singu- 

 lar structure of their placenta, which expands into hard woody processes, which are 

 most commonly hooked. In the form of their embryo they agree AA-ith Bignoniads, but 

 the cotyledons are more fleshy, and their seeds are never winged. From Figworts 

 they are absolutely divided by the absence of albumen, as well as by theu' placental pro- 

 cesses and large fleshy cotyledons. A singular want of development occm's in the 

 calyx of the genera Thunbergia, Mendozia, and Clistax, in which that organ is some- 

 times reduced to a mere obsolete ring, its place being suppHed by bi*acts. Mendozia is 

 also remarkable for its fruit being a 1 -seeded drupe, with crumpled clirysaloid coty- 

 ledons. Mr. Bentham states that the placental processes are sometimes absent; in such 

 cases the embryo can be alone relied upon. 



Fig. CCCCLYIII.— Nelsonia campestris. 1. flowers ; 2. pistil; 3. capsule; 4. cross section of a seed. 



Fig. CCCCLVIII. 



