The flora of the Sonoran province affords a striking example of
the adjustment of vegetal life to an unfavorable environment. The
prevailing vegetation is perennial, of slow growth and of stunted
aspect; and it is not distributed uniformly but arranged in separate
tufts or clusters, gathering into a nearly continuous mantle in wetter
spots, though commonly dotting the plains sparsely, to completely
disappear in the driest areas. Nearly all of the plants have roots
of exceptional length, and are protected from evaporation by a glazed
epidermis and from animal enemies by thorns or by offensive odors and
flavors; while most of the trees and shrubs are practically leafless
except during the humid seasons. Grasses are not characteristic, and
there is no sward, even in oases; but certain grasses grow in the
shadow of the arborescent tufts and in the fields of the farmer ants,
or spring up in scattered blades over the moister portions of the
surface. The arborescent vegetation represents two characteristic
types, viz., (1) trees and shrubs allied to those of humid lands, but
modified to fit arid conditions; and (2) distinctive forms, evidently
born of desert conditions and not adapted to a humid habitat, this type
comprising the cacti and related forms, as well as forms apparently
intermediate between the cacti and normal arborescent type. The various
plants of the district, including those of the distinctive types, are
communal or commensal, both among themselves and with animals, to a
remarkable degree; for their common strife against the hard physical
environment has forced them into cooperation for mutual support. The
tufts or clusters in which the vegetation is arranged express the
solidarity of life in the province; commonly each cluster is a vital
colony, made up of plants of various genera and orders, and forming a
home for animal life also of different genera and orders; and, although
measurably inimical, these various organisms are so far interdependent
that none could survive without the cooperation of the others.[10]
[10] The vital characteristics of the region have been described in
some detail in The Beginning of Agriculture, American Anthropologist,
vol. VIII, 1805, pp. 350-375; The Beginning of Zooculture, American
Anthropologist, vol. X, 1897, pp. 215-230; and Expedition to Seriland,
Science, vol. III, 1898, pp. 493-505.
In Seriland proper, as in other parts of the Sonoran province, a
prevailing tree is the mesquite (_Prosopis juliflora_); on the alluvial
fan of Rio Sonora it grows in remarkable luxuriance, forming (with
a few other trees) a practically continuous forest 20 to 40 feet in
height, the gnarled trunks sometimes reaching a diameter of 2 or
3 feet; over the Rio Bacuache fan and much of the remaining plain
surface it forms the dominant tree in the scattered vital colonies;
and here and there it pushes well into the canyon gorges. The roots
of the mesquite are of great length, and are said to penetrate to
water-bearing strata at depths of 50 to 75 feet; its fruit consists
of small hard beans embedded in slender woody pods. Associated with
the mesquite in most stations are the still more scraggy and thorny
cat-claw (_Acacia greggii_) and ironwood (_Olneya tesota_), both
also yielding woody beans in limited quantity. Similarly associated,
especially in the drier tracts, and characteristically abundant over
the plains portions of Isla Tiburon, are the paloverdes (_Parkinsonia
torreyana_, etc.), forming scraggy, wide-branching, greenbark trees 5 to
15 feet high, and commonly 3 to 10 inches in diameter of trunk. Over
the mountain sides, especially of Sierra Seri and Sierra Kunkaak, grow
sparsely the only straight-trunk trees of the region, rooted in the
rocks to the average number of a few score to the square mile; this is
the palo blanco (_Acacia willardiana_). Associated with it along rocky
barrancas of permanent water supply is a fig tree (_Ficus palmeri_),
which has a habit of springing from the walls and crests of cliffs,
and sending white-bark roots down the cliff-faces to the water 50 or
100 feet below, and which yields a small, insipid, and woody fruit.
Interspersed among the larger trees, and spreading over the intervening
spaces, particularly in the drier and more saline spots, grow a number
of thorny shrubs, much alike in external appearance and habit, though
representing half a dozen distinct genera (_Cassia_, _Microrhamnus_,
_Celtis_, _Krameria_, _Acacia_, _Randia_, _Stegnospherma_,
_Frankenia_, etc.), while considerable tracts are sparsely occupied by
straggling tufts of the Sonoran greasewood, or creosote bush (_Larrea
tridentata_), whose minute but bright green leafage relieves that
prevailing gray of the landscape in which the lighter greens of the
paloverde and cactus stems are lost.
[Illustration: FIG. 4—Beyond Encinas desert—the saguesa.]
Intermingling with the woody trees and shrubs in most stations, and
replacing them in some, are the conspicuous and characteristic cacti
in a score of forms. East of Desierto Encinas, and sometimes west of
it, these are dominated by the saguaro (_Cereus giganteus_), though
throughout most of Seriland the related saguesa (_Cereus pringleii?_)
prevails. The saguaro is a fluted and thorn-decked column, 1 foot to
3 feet in diameter and 10 to 60 feet in height, sometimes branching
into a candelabrum, while the still more monstrous saguesa (figure
4) usually consists of from three to ten such columns springing from
a single root; both are masses of watery pulp, revived and renewed
during each humid season, and both flower in a crown of fragrant and
brilliant blossoms at or near the top of column or branch, and fruit in
fig-like tunas (or prickly pears) during late summer or early autumn.
Ordinarily the saguesa, like the saguaro, is sparsely distributed; but
there is an immense tract between Desierto Encinas and the eastern
base of Sierra Seri in which it forms a literal forest, the giant
trunks close-set as those of trees in normal woodlands. Hardly less
imposing than the giant cactus is the wide-branching species known
as pitahaya (_Cereus thurburi_?), in which the trunks may be ten to
fifty in number, each 4 to 8 inches in diameter and 5 to 40 feet in
height; and equally conspicuous, especially in eastern Seriland, is the
cina (_Cereus schotti_), which is of corresponding size, and differs
chiefly in the simpler fluting of the thorn-protected columns. Both
the pitahaya and the cina flower and fruit like the saguaro, the tunas
yielded by the former being especially esteemed by Mexicans as well
as Indians. Another important cactus is the visnaga (_Echinocactus
wislizeni lecontei_), which rises in a single trunk much like the
saguaro, save that it is commonly but 3 to 6 feet in height and is
protected by a more effective armature of straight and curved thorns;
it yields a pleasantly acid, pulpy fruit, which may be extracted from
its thorny setting with some difficulty; but its chief value lies in
the purity and potability of the water with which the pulpy trunk
is stored. The visnaga is widely distributed throughout the Sonoran
province and beyond, and extends into eastern Seriland; it is rare west
of Desierto Encinas and is practically absent from Isla Tiburon, where
it may easily have been exterminated by the improvident Seri during the
centuries of their occupancy. Most abundant of all the cacti, and less
conspicuous only by reason of comparatively small size, is the cholla
(an arborescent _Opuntia_); on many of the sheetflood-carved plains it
forms extensive thickets 5 to 8 feet high, the main trunks being 2 to 6
inches in diameter, while dozens or hundreds of gaunt and thorn-covered
branches extend 3 to 8 feet in all directions; and it occurs here and
there throughout the district from the depths of the valleys and the
coast well up to the rocky slope of the sierras. It yields quantities
of fruit, somewhat like tunas, but more woody and insipid; this fruit
is seldom if ever used for human food, but is freely consumed by
herbivores. Much less abundant than the cholla is the nopal, or prickly
pear; and there are various other opuntias, often too slender to stand
alone and intertwined with stiffer shrubs which lend them support, and
many of these yield small berry-like tunas. Another characteristic
cactus, widespread as the cholla and abundant in nearly all parts
of Seriland save on the rocky slopes, is the okatilla (_Fouquiera
splendens_). It consists of half a dozen to a score of slender, woody,
and thorn-set branches radiating from a common root, usually at angles
of 30° to 45° from the vertical, and ordinarily reaching heights of 10
to 20 feet.
The pulp masses of the larger cacti, especially the saguaro, saguesa,
pitahaya, and cina, are supported by woody skeletons in the form
of vertical ribs coincident with the external flutings; within a
few years after the death and decay of these desert monsters the
skeletons weather out, and the vertical ribs form light and strong and
approximately straight bars or shafts, valuable for many industrial
purposes; while the slender arms of okatilla are equally valuable, in
the fresh condition after removal of the spiny armament, and in the
weathered state without special preparation.
On many of the higher plain-slopes, especially in eastern Seriland,
there are pulpy stemmed shrubs and bushes, sometimes reaching the
dignity of trees, which present the normal aspect of exogenous
perennials during life, but which are so spongy throughout as to
shrink into shreds of bark-like debris shortly after death. These
are the torotes of the Sonoran province—common torote (_Jatropha
cardiophylla_), torote amarillo (_Jatropha spathulata_), torote blanco
(_Bursera microphylla_), torote prieto (_Bursera laxiflora_), torotito
(_Jatropha canescens_?), etc. These plants grow in the scattered and
scraggy tufts characteristic of arid districts (a typical torote tuft
appears in left foreground of figure 4); they are protected from
evaporation by the usual glazed epidermis, and maintained by the water
absorbed during the humid seasons; but they are thornless and are
protected from animal enemies by pungent odors, and at least in some
cases by toxic juices. Like various plants of the province they are
measurably communal—indeed, the torotito appears to be dependent on
union with an insect for reproduction, like certain yuccas, and like
the cina and (in some degree at least) the saguaro and other cacti.
Along the lower reaches of Rio Bacuache, and in some of the deeper
gorges of Sierra Seri and Sierra Kunkaak, grow a few veritable trees
of moderately straight trunk and grain and solid wood, such as the
guaiacan (_Guaiacum coulteri_) and sanjuanito (_Jacquinia pungens_);
both of these fruit, the former in a wahoo-like berry of medicinal
properties, and the latter in a nut, edible when not quite ripe and
forming a favorite rattle-bead when dry. On the flanks of such gorges
the slender-branched baraprieta (_Cæsalpinia gracilis_) grows up in
the shelter of more vigorous shrubs, its branches yielding basketry
material, while its fruit is a woody bean much like that of the
cat-claw. In like stations there are occasional clumps of yerba mala
or yerba de flecha (_Sebastiana bilocularis_), an exceptionally leafy
bush growing in straight stems suitable for arrowshafts, and alleged
to be poisonous from root to leaf—with inherent probability, since
the plant is without the thorny armature normal to the desert. Along
the sand-washes, especially about their lower extremities wet only in
floods, springs a subannual plant (_Hymenoclea monogyra_) which shrinks
to stunted tussocks after a year or more of drought, but flourishes in
close-set fens after floods; though of acrid flavor and sage-like odor,
it is eaten by herbivores in time of need, and it yields abundant
seeds, consumed by birds, small animals, and men. About all of the
permanent waters not invaded by white men and the white man’s stock
there are brakes of cane or carrizal (_Phragmites communis_?); the
jointed stems are half an inch to an inch in thickness and 8 to 25 feet
in height; the seeds are edible, while the stems form the material for
balsas and afford shafts for arrows, harpoons, fire-sticks, etc., and
the silica-coated joints may be used for incising tough tissues.
The coasts of Seriland, both insular and mainland, are skirted by
zones of exceptionally luxuriant shrubbery, maintained chiefly by fog
moisture. Along the mountainous parts of the coast the zone is narrow
and indefinite, but on the plains portions it extends inland for
several miles with gradually fading characters; this is especially true
in the southern portion of Desierto Encinas, where the fog effects may
be observed in the vegetation 12 or 15 miles from the coast. Most of
the fog fed species are identical with those of the interior, though
the shrubs are more luxuriant and are otherwise distinctive in habit.
On the Tiburon side of gale-swept El Infiernillo, and to some extent
along other parts of the coast, some of these shrubs (notably _Maytenus
phyllanthroides_) grow in dense hedge-like or mat-like masses, often
yards in extent and permanently modeled by the wind in graceful
dune-like shapes. Somewhat farther inland the flatter coastwise zones
of Tiburon are rather thickly studded with shrubby clumps from 6 inches
to 2 feet high, made up of _Frankenia palmeri_ with half a dozen minor
communals; while still farther inland follows the prevailing Sonoran
flora of mesquite, scrubby paloverde, and chaparral (_Celtis pallida_),
etc., only a little more luxuriant than the normal.
Throughout Seriland proper, and especially in the interior valleys
of Tiburon, grasses are more prevalent than in other portions of the
Sonoran province, their abundance doubtless being due to the rarity of
graminivorous animals during recent centuries.