Ch. X.] WATER RESERVOIRS OF PLANTS. 183 



of essential value to each species ; we do not know why 

 white terriers are more subject than darker coloured 

 ones to the attacks of the fatal distemper ; why yellow- 

 fleshed peaches in America suffer more from diseases 

 than the white-fleshed varieties ; why white chickens 

 are most liable to the gapes; or why the caterpillars 

 of silkworms, which produce white cocoons, are not 

 attacked by fungus, so much as those that produce 

 yellow cocoons ? Yet in all these cases, and many 

 others, it has been shown that immunity from disease is 

 correlated with some slight difference in colour or 

 structure, but as to the cause of that immunity we are 

 entirely ignorant. 



At last we reached the summit of the range, which is 

 probably not less than three thousand feet above the 

 sea, and entered on the district of Libertad. Rounded 

 boggy hills covered with grass, sedgy plants and stunted 

 trees, replaced the dry gravelly soil of the Juigalpa 

 district. The low trees bore innumerable epiphytal plants 

 on their trunks and boughs. Many of these are species 

 of TiUandsia, which sit perched up on the small branches 

 like birds. They have sheathy leaves that hold at 

 their base a supply of water that must be very useful to 

 them in the dry season. Insects get drowned in this 

 water, and the plants may derive some nourishment 

 from their decomposing bodies, but I believe the prin- 

 cipal object is to obtain a supply of moisture, as the 

 roots of the plants do not hang down to the ground, 

 like those of many other epiphytes in the tropics, nor 

 are they provided with bulbs like the orchids. Some 

 plants that hold liquids in cup-shaped leaves are simply 

 insect traps, many of them growing in bogs, where the 



