1906 



FORESTRY AND IRRIGATION 



569 



during the latter part of April and 

 early May, supplies the dominant note 

 of coloration in such regious as the 

 desert about Tucson, Arizona. I re- 

 fer to the palo verde, of which there 

 are three species, known as Parkinso- 

 nia micro phylla, P. aculcata and P. 

 Torreyana. According to Sudworth's 

 check list of forest trees of the United 

 States,* the name Cercidium Torreya- 

 num (Wats.) Sargent, is given to the 

 last mentioned species, but for reasons 

 of which I shall speak later it would 

 hardly seem justifiable to separate 

 generically P. Torreyana from the 

 other two species. 



I shall describe first the small leaved 

 palo verde (Parkinsonia micro phylla) 

 (Fig. 23), which is found growing 

 upon the rocky foothills of southern 

 Arizona, California and Sonora, Mexi- 

 co. This plant is as distinctly charac- 

 teristic of this habitat as are the Giant 

 cactus or Suguaro, the Ocotillo (Fon- 

 quieria splendens) and a considerable 

 number of other plants, which in this 

 connection need not be mentioned. It 

 is a small, somewhat irregular tree, 

 ten or twelve feet high, with more or 

 less twisted and contorted limbs 

 clothed with a green bark, this feature 

 being common to all the species, and 

 by which the name "palo verde" is 

 very properly suggested, the name be- 

 ing Spanish for "green tree." The 

 tree usually grows quite plentifully 

 upon the stony hillsides, and in some 

 places, at the time when other vegeta- 

 tion is less conspicuous than usual, 

 has the aspect of a small apple tree, 

 the whole formation looking rather 

 orchard-like. During the early spring 

 the smaller branches, which are lithe 

 tapering twigs, are clothed with bi- 

 pinnate leaves of a rather curious char- 

 acter. The single leaf has a very 

 short rachis, so small indeed as to 

 escape observation at first. From this 

 spring two slender pinnae an inch or 

 so long, which bear six or eight minute 

 orbicular pinnules, scarcely an eighth 

 of an inch in diameter. So small are 

 the leaves that when they fall, as they 



do during the latter part of April, 

 very little difference is to be noted in 

 the general aspect of the tree, although 

 of course this will depend upon the 

 density of the foliage, which varies 

 with different individuals. The leaves, 

 as is true of the Leguminosae in gen- 

 eral, are capable of "sleep" move- 

 ments, the leafllets of the third order 

 folding together upwardly upon the 

 approach of night. When the leaves 

 wither and fall away the pinna as a 

 whole separates from the main, though 

 very small, rachis, the pinnules some- 

 times remaining attached but usually 

 falling away separately. The taper- 

 ing twigs are, when young, slightly 

 pubescent and as they mature their 

 ends develop into thorns. On ac- 

 count of the tapering form of the nu- 

 merous twigs, and their whip-like 

 flexibility, the tree has an exceedingly 

 graceful form. Its delicate evergreen 

 hue always gives its habitat a note of 

 color, even during the dryest seasons 

 of the year when most, if not all, of 

 the remaining vegetation has become 

 more or less neutral in tint. 



The flowers, which are borne in 

 great numbers, are almost radially 

 symmetrical, the only evidence that 

 the flower is of the type of the Legu- 

 minosae being seen in the vexillum or 

 upper petal, which is of a somewhat 

 different form from the rest, being 

 supplied with a longer claw, and white 

 in color, while the rest are light, lemon 

 yellow. The presence of the white 

 petal is sufficient to modify the total 

 color of the flower masses into a rath- 

 er pale, greenish yellow, distinguish- 

 ing it at once by this feature alone 

 from the other species. The dorsiven- 

 trality of the flower is also marked by 

 the unequal stamens and by their posi- 

 tion, and also by the form of the pod, 

 which of course is quite true to the 

 family type. As soon as the insect 

 life in the desert is set in motion by 

 the rising sun the flowers are visited 

 by myriads of insects of all kinds, so 

 that as one stands near a tree their 

 buzzing is very loud. The fruit, which 



*Bulletin No. 17, U. S. Dept. of Agri culture, Division of Forestry, 1898. 



