lyO THE PLANT WORLD. 



pod has undergone a little further growth accompanying 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 ecto- 

 carp, and in this the plant is like the other species, differing from 

 them onlv in degree. The non-adherence of the la\"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. aculcata is a still larger tree, con- 

 fined to a somewhat narrower zone from Yuma, through 

 northern Mexico to Texas. The nearest station to Tucson where 

 it has been found by me is on the western slopes of the Babo- 

 quivari and Coyote ^Mountains, about seventy-five miles away 

 to the southwest, although it may of course occur nearer. It is, 

 like P. Torreyaiia. confined to the washes, which are the river 

 beds, dry for the greater part of the year. When in flower it 

 has nuich the appearance, too, of P. Torreyaiia, 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. iiilcrophyUa. The most striking feature of P. aciilcafa is the 

 leaf which conforms to the type described above for P. iiiicro- 

 f'hylla, but has two pairs of very nnich elongated pinnae, along 

 the marsfins of which are inserted a few small, oblong 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 worthy of note in this connec- 

 tion is the fact that ])lants of this genus are among the few Legu- 

 minosae the seeds of which are provided at maturity with an 

 endosperm, and are therefore described in most systematic works 

 as albuminous. This endosperm is reduced, in the ripened seed, 

 to two horny, translucent layers lying parallel to the cotyledons, 



