18 THE LIFE-HISTORY OF A FERN 
is in a position to nourish itself by taking up from the soil the water and 
salts which it requires, and to elaborate therefrom, and from the carbon- 
dioxide of the air, fresh supplies of organic food. Further, though for 
reasons to be explained later, it frequently is found growing in situations 
where moisture is abundant and the air moist; still it can resist considerable 
drought, and is capable of living under as exacting conditions as any 
ordinary terrestrial plant. As proof of this, cases may be quoted of the 
removal of mature Shield Ferns from a more moist habitat to exposed 
situations, where there is no shade, nor any special supply of water: 
they are found to flourish there permanently; but they show slight 
differences of form from the shade plants: the leaves are more robust, 
of smaller area, and of a paler colour. 
In this power of resisting drought the Shield Fern is by no means 
an isolated exception, nor in any sense an extreme type among Ferns. 
It is a familiar sight in this country to see dry hill-sides covered with the 
common Bracken, and taking no harm from a summer drought. There 
is also a small British Flora of Ferns of dry wall-tops, including such 
species as olypodium vulgare, Asplenium Ruta-muraria, and Ceterach; 
these may be found sometimes with their leaves dried to crispness in 
summer. Abroad there are certain genera, such as JVothochlaena, and 
Jamesonia, and the Miphobolus section of FPolypodium, which are typically 
xerophytic: in other cases isolated species may show special adaptation 
to dry surroundings; for example, Aymenophyllum sertceum which is a 
member of a peculiarly hygrophytic genus. These xerophytic Ferns 
inhabit dry climates, such as the higher Andes: or they are epiphytic 
in habit, and have no access to the water-reservoir of the soil. The forms 
which the xerophytic modification may take are succulence of the smooth 
leaf, with well-developed epidermis, as seen in olypodium nummulariae- 
Jolium, and pilosellordes, andgalso in Flatycertum: or a development of 
a thick felt of hairs may cover the surfaces, as in WViphobolus, Nothochlaena, 
and /amesonia: or of scales, as in Polypodium (Lepicystis) incanum, or 
Asplenium Ceterach: or there may be a xerotropic folding of the pinnae, 
as in lVothochlaena sinuata and ferruginea, and in /Jamesonia. There is 
also a very efficient mode of resisting extreme drought which is not 
shown structurally, viz. the power of retaining vitality after drying up. 
A good example of this is seen in Polypodium (Lepicystis) incanum, which 
grows commonly in Trinidad on tree-trunks, and there shrivels for weeks 
without rain, under a tropical sun; but when moistened again it swells, 
and continues growth. Such vitality is shared in some degree by the Ferns 
of our wall-tops, and is a common feature leading to the survival of 
many other plants, notably among the Bryophytes. Such cases as these. 
quoted will serve to show that a moist habitat is not always a necessity 
for the life of the mature Fern-Plant, and even that Ferns, as a family, 
show a considerable aptitude for resisting extremes of drought. But never- 
theless most Ferns do affect moist situations, while to some, such as 
