FERNS AND THEIR RELATIVES 



But the golden dust serves another purpose. The 

 golden-back, like other ferns, has its pores, or openings, 

 through the leaf-skin, on the underside only. These pores 

 close when the plant is in danger of losing too much water, 

 but much of the time they need to be open to allow the 

 water current to pass out and to assist in breathing, so 

 these pores must not be choked up in any way. Now if 

 you have looked, you know that dew collects on the under 

 side of leaves as well as on the upper, and it remains here 

 longer because the sun cannot reach it. In the sorts of 

 places ferns frequent, leaves are sometimes not wholly dry 

 for months; so you see they need some device to keep 

 their pores from being stopped up by water. The wax- 

 like powder of the golden-back fern, acts like oil on the 

 feathers of water-birds; it prevents the leaves from being 

 wet; the water simply collects in little drops that roll off, 

 and the water current and breathing are not hindered. 



As golden back ferns grow older, a brown powder, 

 too, appears on the lower side of the leaves, first in deli- 

 cate lines, later on perhaps covering the entire surface. 

 The microscope shows that every grain of this brown pow- 

 der is a case full of spores. Some California children know 

 where to find the silver-back ferns. Often young golden- 

 backs are mistaken for silver-backs, but the real silver-back 

 fern is usually larger and hardier than its cousin with the 

 gold dust. The upper surface of the leaf is thick skinned 

 and rather sticky, and the lower side is covered with a 

 silvery powder. 



Have you ever thought what gives the maiden-hair fern, 

 No. i, Fig. 27, its name? Look at its stems. The Greek 

 botanical name, Adiantum, means not wet. Find out if 

 this is a fitting name for the fern. The silvery appear- 

 ance of the leaves under water is due to a layer of air 

 between the leaf and the water. So the maiden-hair fern, 

 as well as the golden-back, knows how to escape being choked 



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