30 Rhodora [FEBRUARY 
the air, whatever the proportion; and aérial roots are equally de- 
void of any power of absorbing vapor." Goebel; from extended 
observation of epiphytes in their native habitats, arrives at the con- 
clusion that absorption of vapor is at any rate not the chief function 
of aérial roots, and approves Duchartre’s opinion. Schimper? is 
equally reserved. 
Up to the present moment too little experimental evidence is avail- 
able to enable one to form a final opinion. It is easy to see possible 
defects in all the methods employed. For example, cut-off roots, it 
may be held, are under essentially unnatural conditions. On the other 
hand if one works with entire plants, transpiration from the shoot and 
assimilation enter as factors in the loss or-gain of weight. Again, the 
vapor-conditions have been under no sort of control (with one excep- 
tion to be mentioned). ‘Thus when Kerner writes as follows, one is 
disposed to attribute the increase of weight to the formation of dew in 
a closed receptacle, the temperature of which was not guarded against 
change. He says: “Ifthe aérial roots of Oncidium sphacelatum are 
transferred from a chamber full of dry air to one full of moist air, they 
take up in twenty-four hours, somewhat more than eight per cent of 
their weight of water." According to Pfeffer, Sachs probably intro- 
duced the same error into his determination of imbibition by dried 
wood. As to aérial roots Pfeffer is silent, as far as I have read his 
latest text; but he notes the very indifferent capacity of vegetable 
tissues in general for acquiring water in the gas-form. 
When my first tests were made, with cut roots partially dried in the 
laboratory and then laid in a moist orchid house, I looked for an addi- 
tion to the weight. The humidity there was usually from Bo to Be, 
I found at the end of twenty-four hours, that the roots were drier and 
lighter than at the beginning. 
A large box was then partly filled with sphagnum. ‘This was 
soaked with water, a glass was placed on the sphagnum, and on this 
were laid the roots. A wet and dry bulb hygrometer, read through 
glass let into the end of the box, gave the humidities. Finally the 
whole was closed in by a sheet of glass, so that the roots had the ad- 
vantage of light. The ventilation was so adjusted that at no time did 
the humidity rise above .95, and varied from this down to a bit 
below .90. 
: Pflanzenbiologische Schilderungen I, p. 188. 2 Pfianzen-Geographie, 1898, p. 343. 
3 Pflanzenphysiolagie, last edition, p. 143. 
