240 GENUS ATRIPLEX. 



widest below the middle. Next to these comes a group in which the original shape of 

 the bracts is cuneate, as best illustrated in A. truncata. But in two of the species {gra- 

 ciliflora, saccaria) the shape is so modified through the development of wing-like margins 

 or appendages that the clue to relationships is supplied only by an exceptional unmodi- 

 fied pair. Close to these species, but perhaps even closer to certain of the Pusillae, 

 is the small group of A. argentea and A. coronata. The latter has fruiting bracts so vari- 

 able in shape that it is often confused with A. cordulata of the pusilla group. A. powelli 

 is placed by itself on the chart because of its semi-dioecious habit, as already noted, and 

 a peculiar shape and lobing of the fruiting bracts. Probably it is not far removed from 

 A. argentea, with which it has in common triple-nerved leaves and other vegetative 

 features. 



Passing over the groups of pentandra and decumhens, which have been already discussed, 

 there remain only the dioecious shrubs. There is abundant evidence that these had 

 their origin far to the south, probably in Mexico. Their migrations to the north and 

 northwest, where they have come to occupy arid saline valleys and foothill slopes, were 

 accompanied by considerable modifications in habit and structure. This has resulted 

 in the development of 11 species and a much larger number of subspecies and minor vari- 

 ations. The most primitive of these, as indicated by their partly herbaceous habit and 

 evident connection with Mexican forms of A. pentandra, are A. barclayana and the group 

 beginning with A. acanthocarpa. The former occupies the area from western Sinaloa 

 and Sonora across Lower California. Evidence as to its former connection with the 

 monoecious herbs is found in its only partial dioecism, in the herbaceous branches, and 

 in having certain subspecies with non-compressed bracts. A. acanthocarpa also is only 

 a subshrub. Although it has evolved certain features of its own, probably it best 

 represents the ancient stock which has culminated in the well-known and polymorphous 

 A. nuttalli, a species which has reached to beyond the Canadian boundary in its north- 

 ward migration. 



The truly shrubby American Atriplexes, that is, those in which even the twigs are 

 woody, now occupy extensive areas in the Great Basin Region and westward, a few of 

 them reaching the saline shores of the Pacific in California. Beginning near A. poly- 

 carpa, which is most like A. barclayana in the character of its bracts, each has developed 

 features so unique that a natural grouping into units larger than the species itself seems 

 impossible. In polycarpa the fruiting bracts and seeds are much reduced in size; in 

 hymenelytra and lentiformis the bracts are strongly compressed; in confertifolia, spinifera, 

 and parryi there appears a spiny habit in addition to certain peculiarities in the bracts. 

 The most divergent species is A. canescens, in which the fruiting body is provided with 

 4 wing-like outgrowths. The connections between these species will be further discussed 

 under the heading of relationships as each is reached in the taxonomic treatment. Their 

 dififerentia-ting characters are illustrated in figure 43 (p. 314). 



CRITERIA FOR THE RECOGNITION OF SPECIES AND SUBSPECIES. 

 Seed. — The most nearly constant character used in the classification of the species of 

 Atriplex is found in the seed. This has to do with the position of the seed within the 

 pericarp. When the former is erect, the radicle points downwards, thus occupying an 

 inferior or basal position in relation to the plumule, which either points upwards or is 

 curved to one side. In a few species {semibaccata, calif ornica, and others) the inferior 

 radicle is turned to one side and takes up a lateral position beneath the tip of the plumule. 

 But in most American species the seed is inverted, so that the radicle points upwards. 

 Such radicles are said to be superior in relation to the plumule, although the latter also 

 curves upwards and its tip is sometimes as high as that of the radicle. In many cases the 

 superior radicle curves inward at tip and thus comes to occupy a position truly superior 



