6. _The Glacial region._--This comprehends all that portion of the Alps that rises above the limit of perpetual snow. But a word of explanation is necessary. The highest parts of the Alps are not covered by one continuous sheet of snow; otherwise we should never see any peaks or crags there. Some are too steep for the snow to rest upon them, and therefore remain bare at heights much greater than the so-called "limit of perpetual snow," and that limit varies considerably. Still this term has a definite meaning when rightly understood. Leaving out of account masses of snow that accumulate in hollows shaded from the sun, the "snow-line" is fairly even, so that on viewing an Alpine range from a distance, the larger patches and fields of snow on adjoining mountains, with the same aspect, are seen to maintain a pretty constant level. [Illustration: ON A GLACIER.] Vegetation becomes scarce in this region, not, as commonly supposed, because Alpine plants do not here find the necessary conditions for growth, but simply for want of soil. The intense heat of the direct rays of the sun (see chapter iii., pages 76-77) compensates for the cold of the night; and it is probable that the greater allowance of light also stimulates vegetable life. But all the more level parts are covered with ice or snow; and the higher we ascend, the less the surface remains bare, with the exception of the projecting rocks which usually undergo rapid destruction and breaking up from the freezing of whatever water finds its way into their fissures. Nevertheless, many species of flowering plants have been found even at the height of eleven thousand feet. It is in this region that plants are found whose true home is in the arctic regions (see chapter ii., pages 64-65). For the sake of those who love ferns, lycopods, and other cryptogamic or flowerless plants, a few words may be said here. Of the polypodies, the beech fern and oak fern are generally common, so is the limestone polypody in places where limestone occurs. Another species (_P. alpestre_) very like the lady fern grows plentifully in many places. The parsley fern, familiar to the botanist in Wales and other parts of Great Britain, is common, especially on the crystalline rocks, and ascends to above seven thousand feet. The holly fern is perhaps the most characteristic one of the higher Alps. It is abundant in almost every district from the Viso to the Tyrol, ranging from about five thousand feet to nearly eight thousand feet. The finest specimens are to be found in the limestone districts. Nestling down in little channels worn out of the rock, it shoots out great fronds, often more than eighteen inches long, which are giants compared to the stunted specimens seen on rockwork in English gardens. _Asplenium septentrionale_ is very common in most of the districts where crystalline rocks abound. The hart's tongue is hardly to be called a mountain fern. The common brake is confined to the lower slopes.