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 



