FERNS, FERN ALLIES, AND FOSSILS. 27 1 



None of these three, however, are hke the ordinary 

 ferns in leaf and general habit. The Wallrue has 

 greatly reduced its leaf surface, and does not, therefore, 

 lose very much water by transpiration in dry weather. 

 Ceterach has a special power of rolling up its leaves 

 so that the dense, yellow-brown woolly scales on their 

 lower surface entirely cover the plant and hinder the 

 escape of water. That most of the family thrive 

 best in moist, shady places has been well shown by 

 Kerner von Marilaun, who compares the dry arid 

 plateaux of Persia with the Blue Mountains in Jamaica. 

 In Persia there is a ten months' dry season, and no 

 ferns are to be found in a district 5,000 square 

 miles in extent. On the Blue Mountains where rain 

 falls on almost every afternoon throughout the year, 

 500 species of fern occur, and fifty different kinds 

 may be found in a hundred paces. 



The reason of their being usually confined to moist 

 places is explained by the structure of any common 

 fern, such as the Male Fern {^Lastrea filix-mas). The 

 leaves are large and much divided in a feathery (or 

 pinnate) manner. They extend over a large surface. 

 Each leaf is clearly intended to form a segment in 

 the broad circle of drooping fronds, which will intercept 

 as much as possible of the scanty light that falls 

 upon it. But these leaves are thin and soft in 

 texture ; they show scarcely any protective arrange- 

 ments against sun, wind, or drought. The epidermis 

 is not, or is only very slightly thickened, and it con- 

 tains green chloroplasts, showing that it is scarcely at all 

 specialised for the defence of the frond. 



When young the leaves are very carefully protected. 

 The whole frond is rolled up into a spiral coil, and 

 each of its leaflets is also rolled towards and under 



