1900] Leavitt,— Relation of plants to moisture 67 
trunks of trees. .Szricta pulmonaria is a large thalloid, or flattened 
form clinging loosely on rough bark. “Wet weight ” was taken after 
soaking in water and allowing to dry until no water appeared on the 
surface, ‘dry weight" after heating. 
Species. Wt. at Time Humidity, ' Loss. Gain. 
start. elapsed. etc. Gms. Gms. 
Gms. 
Usnea barbata. 3.320 4 days. .93-.95 -800 
2.520 4 days. In lab. -485 
2.035 3 days. -93—95 -245 
Wet weight, 4.650 
Dry weight, 1.680 
Sticta pulmonaria, 2.320 I day. .93-.95. 147 
ál 25 days. Dr 250 
Wet weight, 5.240 
Dry weight, 1.860 
One more table is added, designed to give a rough idea of the rel- 
ative store of water retained after exposure in an atmosphere nearly 
saturated with water-vapor. 
Species. Percentage of water Percentage after Time in 
when well moistened. stay in box. days. 
Usnea barbata, .64 .26 +3 
n ^ 53 .20 32 
Cladonia rangiferina. .20 +7 
C. cristatella. 2E Tit 
C. pyxidata, 63 .22 -10 
C. furcata. .69 .36 -7 
Baeomyces roseus, 75 .20 -9 
Parmelia caperata, 54 .24 +3! 
Sticta pulmonaria. .66 127 +25 
The positive and negative signs in the last column indicate whether 
the lichen was originally dry and had gained weight, or moist and had 
lost weight. 
The figures are not presented as in any sense physical constants, 
but merely to show in a general way the relation of some lichens to a 
possible source of replenishment of their water supply in cases of the 
failure of atmospheric precipitation. After long exposure to very moist 
air the lichens possessed, it will be seen, but a low proportion of 
water. In outdoor humidities this would be still less considerable, 
and the quantities of water received in the form of vapor must be next 
to nothing in very dry weather. On the other hand in damp weather 
1 Two tests, three days losing, three days gaining; result about the same. 
