1900] Leavitt, — Relation of plants to moisture 65 
tus, lacking a plant, was also used and weighed in the same way as the 
fir.t. This suffered slight changes of weight, by evaporation from the 
rubber, but these were always trifling compared with the losses of the 
other contrivance. 
The experiment was repeated with another young Dendrobium 
plant having three small leaves and 96 inches of roots. No calcium 
chloride was used. 
The tests lasted four, five, and six days respectively. Every suc- 
cessive weighing showed a somewhat diminished weight. In three 
days, through the 28 inches of slender and dry roots of the first plant 
about .o7 gram was lost. The second plant lost about the. same 
amount in like condition in the same time. In both cases the leaves 
were transpiring ; and the test was carried on until the second plant 
was plainly suffering for want of water, although the air about the roots 
was very nearly (.95) saturated with water-vapor. 
These two Dendrobiums, with still a third, have been hung unpro- 
tected in a greenhouse where the atmosphere is well charged with 
moisture, and from time to time their weights have been determined. 
No. 1, though not watered for seven weeks, is green and healthy. ‘The 
stem is somewhat shrivelled. It has lost .57 gram weight in the last 
20 days, a little more than one-eleventh of the present gross weight. 
The others also slowly decline in weight. 
It remains to be proved conclusively that the roots of any orchids 
possess a special condensing power. The fitness of the velamen for 
such a function may well be classed with the “ evident” adaptations. 
TiLLANDSIA. <A piece of T. usneoides, which had hung in the green- 
house, was tested in the damp-box in the same manner as the orchid 
roots. ‘Tips here and there finally dried up and died, and the whole 
lost weight continuously for a long time. ‘This plant probably has no 
power of absorbing water in the gas form. | 
Mosses. The reservoir-cells of Sphagnum and ZLeucoóryum in some 
respects resemble the tracheid-like elements which give the covering of 
the orchid root its peculiar spongy character. Kerner attributes to 
them the same function, viz., the appropriation of water-vapor from 
the atmosphere at those seasons of the year when the supply from the 
ground is cut short by drought. 
Leucobryum, sp. December 19, the moss was gathered in a dryish. 
condition. It was left in the laboratory for two days, when it seemed 
very dry to the touch. It was then weighed and put into the box. 
b ls 
