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334
NOTES OF A BOTANIST
of herbs exist there, which, burying themselves deep in the earth,
survive through the long periods of drought to which they are
subjected. Some of the smaller medanos, especially those under
the lee of a low ridge of land, may be seen to be capped with
snowy white, contrasting with the yellowish or greyish white
which is the ordinary colour of the sand, and yet at a short
distance liable to be taken for sand a little whiter than common.
The whiteness, however, is that of the innumerable short cylindrical
spikes of an Amarantacea, whose stems, originating from
beneath the medano, ramify through it, and go on growing so as
to maintain their heads always above the mass of sand, whose
unceasing accumulation at once supports and threatens to overwhelm
them.
The other two herbs of the desert are known to the natives,
the one as Yuca del monte or Wild Yuca, the other as Yuca de
caballo or Horse Yuca, from their having roots like those of the
cultivated yuca (Manihot Aypi\ or not unlike parsnips, but three
times as large. Both roots are edible, and the former is sometimes
brought to market at Piura when the common yuca is
scarce. The Yuca de caballo is too watery to be cooked, but is
sometimes chewed to allay thirst by the muleteers and cowherds,
who detect its presence by the slightest remnant of the dried
stump of a stem ; for both kinds maintain a purely subterranean
existence during many successive years, and only produce leafy
stems in those rare seasons when sufficient rain falls to penetrate
to the roots. A few animals that roam over the desert, such as
goats, asses, and horses, obtain a scanty supply of food and drink
from these yuca roots, which they scrape out with their hoofs.
The fruit of the Yuca de caballo may freemen tly be seen blowing
about the desert, looking more like a pair of very long hooked
bird's claws than anything vegetable. It is an elongated capsule
with a fleshy pericarp (incorrectly described as a drupe), termin-
ating in a beak several inches long, and when ripe splitting into
two valves, which remain united at the base and curl up so as to
resemble claws or ram's horns. At Piura it is known by the not
very apposite name of espuelas or spurs. In Mexico the fruit
of an allied species is called Una del diablo or Devil's Claws.
The Yuca de caballo is a Martynia, of the family of Gesnerere
(or, according to some, of Cyrtandracere). I was fortunate
enough to see a single plant of it with leaves and flowers in 1863,
near the river Piura, on ground which the inundation had barely
reached, but had sufficed to cause the root to shoot forth its
stems, which spread on the ground, branching dichotomously, to
the distance of a yard on all sides. The roundish leaves, clad
with viscid down, are lobed much in the same way as those of
some gourds, but the large sweet-smelling flowers are like those
of a foxglove.