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 OF CENTRAL AFRICA. 271

and where their union is more intimate, their existence is still evident from the want of correspondence, and conse- quent decussation of their areolae. The lamellae, which arc usually very thin and transparent, have their surface divided into areolae, in different genera of very different forms, some of which may, with sufKcient clearness, be described. In many cases, no other appearance exists; in some, however, the axis of the septum resembles either a single nerve, or two distinct parallel nerves ; and from this axis, whether formed of one or two nerves, tubes having the appearance and ramification of the veins of a leaf, and which generally terminate within the margin, not unfre- quently proceed. This is remarkably the case in Earsctia, as I here propose to limit that genus ; the central vessels in both its species being closely approximated, so as to form a single cord, extending from the apex to the base of the septum, and the veins being numerous and uncommonly distinct. Approaches more or less manifest to this struc- ture of Earsetia exist in several other genera, as in Parrya, Savignya, andKoniga. But in this last-mentioned genus [~no the nerve, which originates, as in all cases, at the apex, hardly extends, even in the polyspermous species, beyond the middle of the septum, and the veins which are nuich less distinct, are descendent.

As far as my observations on this subject at present ex- tend, I expect, with great confidence, uniformity in the structure of the septum of strictly natural genera, and in many cases, though certainly not in all, I have found a resemblance in this respect in more extensive groups. Thus Draba, Arabis, and Au1)rietia, agree in having amorphous areola?, bonnded by ficxuose tubes or lines ; Avhile Alyssum, Eertcroa, and Tibigia, have narrow linear areolcc, bounded by parallel or slightly arched lines. Capsella bursa differs from Thlaspi and yKthioncma, as Draba from Alyssum, and agrees with Lcpidium procumbens, Linn., improperly re- ferred to liutchinsia, and which equally has incumbent cotyledones. Cochlearia differs in like manner from Ker- nera. And numerous other examples of the same agree- ment in nearly related plants, and of differences where the

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