o
howl, and I wouldn't for worlds deprive them of the privilege.) Some
familiar analogies will help us to see the utility of this arrangement.
Everybody knows our common English stone-crops--or if he doesn't he
ought to, for they are pretty and ubiquitous. Now stone-crops grow for
the most part in chinks of the rock or thirsty sandy soil; they are
essentially plants of very dry positions. Hence they have thick and
succulent little stems and leaves, which merge into one another by
imperceptible gradations. All parts of the plant alike are stumpy,
green, and cylindrical. If you squash them with your finger and thumb
you find that though the outer skin or epidermis is thick and firm, the
inside is sticky, moist, and jelly-like. The reason for all this is
plain; the stone-crops drink greedily by their roots whenever they get
a chance, and store up the water so obtained to keep them from
withering under the hot and pitiless sun that beats down upon them for
hours in the baked clefts of their granite matrix. It's the camel trick
over again. So leaves and stem grow thick and round and juicy within;
but outside they are enclosed in a stout layer of epidermis, which
consists of empty glassy cells, and which can be peeled off or flayed
with a knife like the skin of an animal. This outer layer prevents
evaporation, and is a marked feature of all succulent plants which grow
exposed to the sun on arid rocks or in sandy deserts.

The tendency to produce rounded stems and leaves, little
distinguishable from one another, is equally noticeable in many seaside
plants which frequent the strip of thirsty sand beyond the reach of the
tides. That belt of dry beach that stretches between high-water mark
and the zone of vegetable mould, is to all intents and purpose a
miniature desert. True, it is watered by rain from time to time; but
the drops sink in so fast that in half an hour, as we know, the entire
strip is as dry as Sahara again. Now there are many shore weeds of this
intermediate sand-belt which mimic to a surprising degree the chief
external features of the cactuses. One such weed, the common
salicornia, which grows in sandy bottoms or hollows of the beach, has a
jointed stem, branched and succulent, after the true cactus pattern,
and entirely without leaves or their equivalents in any way. Still more
cactus-like in general effect is another familiar English seaside weed,
the kali or glasswort, so called because it was formerly burnt