THE EPIPHYTES 157 



water causing the grey colour to turn green. 

 Water rapidly passes into the cells of the hair 

 and thence is transmitted to the leaf. As 

 the air becomes dry again the hair gradually 

 flattens down, and its thick outer wall serves 

 as an additional barrier to prevent undue 

 loss of water from the leaf. 



It is obvious that tillandsias, and other 

 plants which, like them, grow on the branches 

 of trees, must be largely dependent on some- 

 what casual sources for the small supply of 

 mineral salts which are required for their 

 subsistence. Most of it reaches them in the 

 form of dust, or as vegetable detritus of 

 various kinds. Inasmuch as the leaf -hair, 

 like the root-hair, can only absorb substances 

 already in solution, it becomes a question 

 of some interest to ascertain whether the 

 salts really do pass into the plant in this 

 way, and if so whether the absorbent hairs 

 of the epiphytic tillandsias differ in this 

 respect from their near relatives which still 

 grow in the ground. 



It has been ascertained that salts are so 

 taken in by the epiphytic species, but, as 

 might be expected, not necessarily or com- 

 monly so by the rest. The Pine-apple, a 

 terrestrial plant related to Tillandsia, possesses 

 hairs similar to those of the latter plants, on 

 its leaves. They are able to absorb water, 

 but the salts dissolved in it do not pass in, 

 for they cannot traverse the protoplasmic 

 lining of the cells. 



