a stout
dagger-like point, as with the agave, or so-called American aloe;
sometimes they are reduced to mere prickles or bundles of needle-like
spikes; sometimes they are suppressed altogether, and the work of
defence is undertaken in their stead by irritating hairs intermixed
with caltrops of spines pointing outward from a common centre in every
direction. When one remembers how delicately sensitive are the tender
noses of most browsing herbivores, one can realize what an excellent
mode of defence these irritating hairs must naturally constitute. I
have seen cows in Jamaica almost maddened by their stings, and even
savage bulls will think twice in their rage before they attempt to make
their way through the serried spears of a dense cactus hedge. To put it
briefly, plants have survived under very arid or sandy conditions
precisely in proportion as they displayed this tendency towards the
production of thorns, spines, bristles, and prickles.

It is a marked characteristic of the cactus tribe to be very tenacious
of life, and when hacked to pieces, to spring afresh in full vigour
from every scrap or fragment. True vegetable hydras, when you cut down
one, ten spring in its place: every separate morsel of the thick and
succulent stem has the power of growing anew into a separate cactus.
Surprising as this peculiarity seems at first sight, it is only a
special desert modification of a faculty possessed in a less degree by
almost all plants and by many animals. If you cut off the end of a rose
branch and stick it in the ground under suitable conditions, it grows
into a rose tree. If you take cuttings of scarlet geraniums or common
verbenas, and pot them in moist soil, they bud out apace into new
plants like their parents. Certain special types can even be propagated
from fragments of the leaf; for example, there is a particularly
vivacious begonia off which you may snap a corner of one blade, and
hang it up by a string from a peg or the ceiling, when, hi, presto!
little begonia plants begin to bud out incontinently on every side from
its edges. A certain German professor went even further than that; he
chopped up a liverwort very fine into vegetable mincemeat, which he
then spread thin over a saucerful of moist sand, and lo! in a few days
the whole surface of the mess was covered with a perfect forest of
sprouting little liverworts. Roughly speaking, one may say that every
fragment of every organism has in it the power to re