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. 
