192 M. Brongniart on the different Floras which 



close affinity with others. Having the lowest spikelets composed 

 of alternate spicula instead of crowded, is scarcely a sufficient 

 distinction between this and axillaris, and I have a specimen in 

 which there is an attempt to cluster in Bcenninghausiana, while 

 it preserves its other characters. The fruit can scarcely be ad- 

 mitted as evidence when immature ; it undergoes many changes 

 in form before it ripens, and the young fruit in axillaris and 

 remota is identical with it. 



The roughness reaches below the middle, it is said, in the peri- 

 gonium of Bcenninghausiana ; so it does in axillaris when very 

 young, and the thickening of the fruit and consequent forming of 

 the beak appear to be from below upwards, where the embryo 

 is first placed. 



It may be a hybrid produced from the impregnation of axil- 

 laris by the pollen of another Car ex, as remota. Be this as it 

 may, it is very singular that it does not come to perfection, and 

 this fact strengthens the idea that it may be a hybrid. 



I think we are perfectly justified in regarding it as a variety 

 of axillaris, unless, were it ever to mature, it should prove dif- 

 ferent. 



The last Carex I shall notice is an alpine one placed by Mr. 

 Babington as a distinct species under the name of Carex Per- 

 soonii. This too has evidently been examined in an immature 

 state, as Mr. Babington is usually particular in mentioning the 

 form of the nut, which he has omitted here. It turns out in fact 

 to be identical with Carex curta ; its spikelets as they ripen are 

 becoming from oblong, roundish-elliptical, on account of the 

 spreading of the fruit. The perigonium has become longer than 

 the membranous glumes, and has taken the exact form of that in 

 curta, the split beak having become an emarginate one, and the 

 nut elliptical. This is an illustration of what I referred to before, 

 and shows how necessary it is to have a mature plant before we 

 write a description. 



XVIII. — Chronological Exposition of the Periods of Vegetation and 

 the different Floras which have successively occupied the surface 

 of the Earth. By M. Adolfhe Brongniart. 



;7Ii.! 



' 'Vr' " [ ' rContinued from p. 85.1 

 ill 'J'^i'^<'^"^*g.'XlNG60M OF THE GyMNOSPERMS. 



During the preceding periods, and especially during the carbo- 

 niferous, the acrogenous Cryptogams predominated, and the 

 gymnospermous Dicotyledons, less numerous, presented them- 



