1 8 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 Polypodiuin rulgare, Aspleniuin Ruta-mitraria, and Ceterach ; 

 these may be found sometimes with their leaves dried to crispness in 

 summer. Abroad there are certain genera, such as Nothochlaena, and 

 famesonia, and the NipJiobolus section of Polypodium, which are typically 

 xerophytic : in other cases isolated species may show special adaptation 

 to dry surroundings ; for example, Hymenophylhim sericeitm 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 Polypodium nummulariae- 

 folium, and piloselloides, and also in Platy cerium : or a development of 

 a thick felt of hairs may cover the surfaces, as in Niphobolus, Nothochlaena, 

 and Jamesonia : or of scales, as in Polypodium (Lepicystis} incanum, or 

 Asplenium Ceterach : or there may be a xerotropic folding of the pinnae, 

 as in Nothochlaena 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 



