570 



FORESTRY AND IRRIGATION 



December 



develops rapidly during- the early sum- 

 mer, consists of a papery pod bearing 

 from one seed to a half dozen, each 

 one of which is separated from its 

 neighbor by a marked constriction of 

 the pod, which at its outer end is con- 

 tinued into a slightly curved, rather 

 long beak. The whole of the pod 

 when ripe splits into two layers, the 

 inner of which consists of a narrow 

 strap of tissue which extends through- 

 out the whole length of the pod, and 

 is no wider at any point than is the 

 constriction which occurs between the 

 seed chambers. The outer layer, on 

 the other hand, is the part which gives 

 the form to the pod, and it will be seen 

 therefore that during the ripening the 

 inner layer or endocarp takes no part 

 in the secondary enlargement of the 

 pod, which accompanies the enlarge- 

 ment of the seeds. This feature dis- 

 tinguishes this species and P. aculeata 

 from P. Torreyana, in which latter the 

 whole pod develops without constrict- 

 ing between the seeds, and has a form 

 very like that of an ordinary pea pod. 



P. Torreyana, known also as the 

 green-barked acacia, is a larger tree 

 than P. microphylla, with, however, 

 the same general habit of growth, 

 save that the branches are somewhat 

 less twisted and the terminal twigs 

 longer. This tree grows in "washes," 

 and apparently needs more water than 

 its neighbor. It flowers very abun- 

 dantly, the tree becoming a mass of 

 brilliant yellow when in full bloom. 

 The twigs are usually armed with 

 short thorns, which are very short, 

 leafless branches. The leaflets of this 

 species are considerably larger, and 

 the prominent petal, which in P. mi- 

 crophylla is white, is here yellow dot- 

 ted with red, though slightly different 

 in form from the rest of the petals. 

 The pod of P. Torreyana, superficially 

 regarded, differs materially from that 

 of the other species, inasmuch, as 

 above pointed out, there is no constric- 

 tion between the seeds, or at any rate, 

 very little, and this not constant. The 

 ovary wall is papery, however, and 

 while similar in general appearance to 

 that of the common pea pod, differs 



from it in the splitting of the outer and 

 inner layers of the wall. In the region 

 of the seed, where the pod has under- 

 gone a litte further growth accompa- 

 nying the growth of the seed, the inner 

 layer, or endocarp, is found to have 

 been arrested in its development, and 

 so is not as wide as the ectocarp, and 

 in this the plant is like the other spe- 

 cies, differing from them only in de- 

 gree. The non-adherence of the lay- 

 ers of tissue of the ovary wall results 

 in a lack of tension which is to be 

 found in many other species and which 

 is related to the expulsion of the seeds, 

 the setting free of which in these 

 plants is accomplished by the mere 

 splitting of the pod without any 

 marked twisting of the fruit wall. 



The third species, P. aculeata, is a 

 still larger tree, confined to a some- 

 what narrower zone from Yuma, 

 through northern Mexico to Texas. 

 The nearest station to Tucson where 

 it has been found by me is on the west- 

 ern slopes of the Baboquivari and 

 Coyote Mountains, about seventy-five 

 miles away to the southwest, although 

 it may of course occur nearer. It is, 

 like P. Torreyana, confined to the 

 washes, which are the river beds, dry 

 for the greater part of the year. When 

 in flower it has much the appearance, 

 too, of P. Torreyana, the flowers being 

 wholly yellow with red markings on 

 the upper petal, which turns brown 

 with age. The pod is very similar in 

 structure to that of P. microphylla. 

 The most striking feature of P. acu- 

 leata is the leaf which conforms to the 

 type described above for P. micro- 

 phylla, but has two pairs of very much 

 elongated pinnae, along the margins 

 of which are inserted a few small, ob- 

 long leaflets, so small that they are 

 scarcely noticeable at a short distance. 

 The rachis becomes a sharp thorn, and 

 on the rapidly growing shoots the 

 stipules are also in the form of spines. 

 The pinnae are green and strap-shaped 

 and sometimes reach the length of one 

 and a half feet and being persistent 

 they give by their pendulous habit a 

 graceful, willow-like form to the tree. 



Another matter that is especially 



