STRUCTURE.] IN SPURGEWORTS, ETC. 49 



is to be found in Leguminosae, Soapworts, Anonads, Dil- 

 leniads, and many other plants. In this case, I have fre- 

 quently seen a true aril, but never an arillary expansion of 

 the edges of the exostome. At other times, over one or two 

 exterior plates of the testa, which are cartilaginous, or crust- 

 aceous, we find a layer, more or less thick, of parenchyma, in 

 which the raphe and its ramifications are visible. This 

 exterior layer, often described by Gsertner, under the name 

 of epidermis, and considered in Euphorbiacea? as an aril, by 

 M. Roeper, is characteristic of the seeds of entire natural 

 orders, as of Euphorbiacea3, Malvaceas, Butneriacese, Myris- 

 ticacese, Tiliacese, Polygalacese, Hypericacese, Violacese, Lina- 

 cea3, Thymelacese, &c., &c.; and it is on these seeds that the 

 expansions of the micropyle, which have often been con- 

 founded with those of the funicle, have been found. Between 

 these two states of the testa, which are sometimes perfectly 

 well marked, a number of intermediate stages are found, 

 which make them run one into the other ; Rhamnus, among 

 many other examples, according to the correct observation of 

 M. Ad. Brongniart, unites them both. Indeed, when I say 

 that the expansion of the exostome is peculiar to those seeds 

 which have a visible raphe on their outside, I do not mean 

 that a true aril cannot exist on their testa, since Bixa, the 

 seeds of which are covered with pulp, has, nevertheless, a 

 rudimentary aril. 



The seeds of Euphorbia, of Ricinus, and of many other 

 Spurgeworts, have at the side of their point of attachment 

 a fleshy, lenticular, or hemispherical excrescence, which, at 

 an early period, attracted the attention of observers. Without 

 saying anything as to its nature, Adanson described it as a 

 fleshy tubercle ; and Gaertner, after him, as a thick spongy 

 hilum. This error of the German carpologist was soon fol- 

 lowed by one of a more serious nature. Some botanists, no 

 doubt undecided as to the value of the word aril, applied it 

 to the caruncle of Euphorbia. M. Mirbel, however, in his 

 excellent memoir on the ovules of Euphorbia Lathyris, cleared 

 up all doubts as to the nature of this excrescence, and clearly 

 showed it to be nothing but the thickened edge of the exos- 

 tome. If we compare this fact with that we have already 



VOL. II. E 



