ORIGIN AND DEVELOPMENT OF THE SECTIONS. ZdV 



The rosea group, consisting of three important species introduced from Europe, has 

 had a development corresponding to that of patula. But here the fruiting bracts, instead 

 of remaining soft have become more hardened and cartilaginous than in any other 

 group. For this reason the name Sclerocalymna was proposed for the section by 

 Ascherson. The similarity to other sections is expressed, on the other hand, by the 

 taxonomically equivalent name of Obionopsis Lange. The shape as well as the degree 

 of induration of the fruiting bracts is closely approximated in some forms of A. argentea 

 and other members of Obione. 



The most primitive American representatives of the subgenus Obione are A. dioeca 

 and A. monilifera. They belong to the so-called genus Endolepis Torrey, which, how- 

 ever, is better received as a section of Airiplex. A much reduced but evident perianth 

 is present in the pistillate flowers of these species, as also in those of the closely related 

 A. phyllostegia. In this last, however, the perianth sometimes is completely suppressed, 

 as it is in all of the succeeding species of Airiplex. 



In order to arrange the remaining species in their proper phylogenetic sequence, it is 

 necessary first to determine the evolutionary significance of the characters which dif- 

 ferentiate them. Of these, the development of dioecism and of the shrubby habit seem 

 to be the most important. There are no dioecious species in Euatriplex. In Obione 

 the tendency toward a complete separation of the sexes is noted at a number of places, 

 especially in the species phyllostegia and powelli. Complete dioecism is attained, how- 

 ever, only in the truly shrubby species and two herbaceous ones, namely, decumbens and 

 matamorensis. The nearly complete parallelism between these two characters is assumed 

 to justify their use as criteria for the assembling of the large number of species into two 

 natural groups — the group of monoecious herbs and the group of dioecious shrubs. 

 The two notable exceptions just mentioned form an intermediate group marked also 

 by its opposite leaves, an almost unique feature in the genus. The transition between 

 the group of monoecious herbs to that of the dioecious shrubs is found in some members 

 of the pentandra subgroup with woody basal portions and in "shrubby" species {bar- 

 clayana, acanthocarpa, corrugata) which are sometimes herbaceous nearly to the base. 

 Of these latter, A. barclayana is but imperfectly dioecious, since at least a few fruiting 

 bracts may be found on many of the pistillate plants. The connection between the 

 pentandra group and barclayana is suggested also by the turgid bracts in certain forms 

 of each and by striking similarities in other features. The trait of monoecism persists 

 also in A. lentiformis, a truly shrubby species, for, although the individuals of this are 

 for the most part either chiefly staminate or chiefly pistillate, plants are not rare on 

 which the sexes are about equally represented. Occasional monoecism is found also in 

 other shrubby species. 



The derivation of the dioecious shrubby Atriplexes from a stock close to the pentandra 

 group, as just proposed, may appear to start them off too far along in the phylogenetic 

 sequence. It should be noted, however, that while this gives them a high position on 

 the chart of relationships (fig. 29, p. 238), it is possible that pentandra itself may be 

 more primitive than might be assumed from its location. The evolutionary line, as 

 charted in the diagram, does not pass through any group above Endolepis, so that the 

 pentandra group may be considered, if so desired, as no higher in the scale than the 

 others, each of which terminates a divergent line. A. pentandra may be even basal to 

 these others, as its center of distribution in Mexico would seem to indicate, but the 

 tendency to an obovate shape of the fruiting bracts, which is a reversal of the usual 

 leaf and bract shape, and the separation of the sexes into different inflorescences, which 

 presages the dioecism of the shrubby species, furnish evidence opposed to this view. 



Atriplex pusilla and its immediate allies form a natural group within the larger assem- 

 blage of monoecious herbs of the subgenus Obione. All are annuals with fruiting bracts 



