There are several reasons why these plants took so long to dry up. To begin with, they have inside their stems and leaves certain substances which hold water and delay its escape. Moreover their extraordinary shapes are of very great assistance. They prefer globular, round, circular, pear-shaped, or cylindrical forms. Suppose you were to cut such a round mass into thin slices and lay them out flat, it is quite clear that they would cover a much greater surface. Thin leaves also, if squashed up into a round ball, would have a very much smaller surface. The water can only escape from the surface exposed, so that these condensed round balls and fleshy columns have far less water-losing surface than ordinary leaves. As a matter of fact, it was found by calculation that the surface of an Echinocactus was 300 times less for the same amount of stuff as that of an Aristolochia leaf. If the actual loss of water from the Echinocactus, as found by experiment, was reckoned as one unit per square inch, then the amount of water lost from a square inch of the Aristolochia was no less than 5000 units! This shows that these odd, outrageous shapes of Prickly Pears, Cacti, and other succulents are an extraordinary help to them. We have already pointed out in a previous chapter how necessary their spines and prickles are if they must resist rats, mice, camels, and other enemies. What we may call the "hedgehog" type of plant is also very common in desert countries. There are many woody little, much branched, twiggy shrublets, which bristle all over with thorns and spines. They are not at all fleshy, but do with the least conceivable amount of water. Another striking characteristic of the desert flora is noticed by every one. Almost every plant is clothed either in white cottonwool, like the Lammie's Lug of our gardens, or else in grey hairs. The general tint of the landscape is not green, but it is rather the colour of the soil silvered over by these grey-haired plants. The reason of this is, of course, quite easy to understand. We put on a thick overcoat if we are going to walk in a Scotch mist, to keep out the moisture. These plants cover themselves with hairs or cottonwool to keep the moisture inside. It does not escape easily through the woolly hairs on the skin. One very strange plant should be noticed here. This is the Iceplant (_Mesembryanthemum cristallinum_). Every part of it is covered with little glittering swellings which shine in the sun like minute ice crystals. The swellings contain a store of water, or rather of colourless sap, which makes it able to exist in dry places. Dr. Ludwig says that a torn-off branch remained quite fresh for months on his study table. It is probable that these peculiar pearl or ice-like swellings also focus the sunlight, acting like lenses, upon the inner part of the leaf, but that is not as yet fully understood.