94 



in patches with moss, on the dechvity helow the great sandstone 

 parapet. But wherever found it can always be readily distin- 

 guished from P. vulgar c by the much smaller, coriaceous, ob- 

 long to triangular-lanceolate fronds, covered on the under sur- 

 face with a thick grayish brown scurf. This scurfy coating plays 

 an important part in connection with the drought resisting quali- 

 ties of the plant. Viewed under a good hand lens it is seen to 

 consist of a multitude of minute gray scales, each with a dark 

 brown spot in the center. These scales cover the stomata or 

 transpiration pores on the l)ack of the frond, and when there is a 

 dearth of moisture they retard evaporation from the surface, 

 thus causing the frond to curl over on its face, exposing to the 

 sun and air a scale armor that checks evaporation and thus 

 enables the plant to preserve its vitality without water for an 

 astonishing length of time. The agency of the scales in this im- 

 portant function was tested in a number of experiments by re- 

 moving them* from one or more healthy frond.s, leaving others 

 on the same rootstock in their normal condition, and noting the 

 relative time of wilting or recovery in each case. When fresh 

 fronds were deprived of moisture, the denuded frond, other 

 things being equal, always wilted more quickly than the others ; 

 but when conditions were reversed and dry specimens placed in 

 water on a bright, clear day, the naked frond, on account of more 

 rapid transpiration, recovered more slowly. In one experiment, 

 where only one half of a vigorous, healthy frond was denuded, 

 the two sides showed very little difference in warm, rainy weather, 

 but when the atmosphere was dry the naked side was first to 

 show signs of wilting, though the difiference was not so marked as 

 in the case of separate fronds. 



To decide how long the polypodium can tolerate continuous 

 drought without losing its vitality, a number of experiments were 

 carried on at different times and places. In these, each of the 

 specimens used was gathered with its mossy substratum intact, 

 and kept in a dry place, where portions were separated from it at 

 intervals and placed in water as long as any of them continued 



* This can l)e done without injury to the epidermis, by gently scraping ofT 

 the scales with a sharp knife while in the expanded state. 



