6 Fritsch . — The Moisture Relations of Terrestrial Algae. I. 
generally attained about 8 a.m. 1 ; this was followed by a rapid drop to 
between 40 and 50 per cent., a point reached between 2 and 4 p.m. For 
some two or three hours the air remained in this condition, but usually 
about 5 or 6 p.m. a rise of moisture-content took place, and this increase 
continued steadily till the maximum was reached about 8 a.m. the next 
morning. On damp or rainy days the pen of the hygrograph traced out 
an almost straight line, usually ranging about 80 per cent. In all proba- 
bility these data are typical of the south of England for this time of the 
year. A similar sequence seems frequent in winter, the maximum humidity 
being attained soon after daybreak, but the rise in the afternoon commences 
almost immediately after the minimum is reached. 
With the falling humidity, soon after 8 a.m., 1 all the various materials 
investigated give off moisture rapidly, but this is arrested soon after 
midday and for part of the afternoon the weight often remains almost 
constant. Soon after 5 p.m., however, an increase is observed, and this 
continues steadily until dusk. No further weighings were made after that 
time, but there is evidently a very considerable absorption of moisture over- 
night, since the weight at dusk is always far less than that recorded in the 
early morning (cf. Table V). When the middle of the day is dry as 
compared with the morning, a considerable daily range is observed, whilst 
when the humidity of the air remains high the daily range is small. There 
may thus be a considerable diurnal range in moisture-content, but losses 
sustained on a dry day are often more than made good overnight. When 
the humidity of the air remains high for several days in succession, a steady 
increase in weight may be observed day after day, implying a very marked 
absorption of moisture (cf. Table IV). 
Table IV. 
Comparison of weights of diverse materials in the early morning of successive 
days (increase indicated by 4- , decrease by — ; in all cases the increments 
are expressed as percentages of the total range observed during the 
period of the experiment 2 ). 
Date. 
Pleurococcus 
on thin 
layer of soil. 
Soil with 
moss pro- 
tonema and 
Hormidium. 
Holly bark 
with 
Pleurococcus. 
Pinus 
bark 
with 
ditto. 
Remarks. 
Aug. 
16 
— 2 
~3| 
+ 22-4 
+ 4 
-If 
17 
+ I2| 
+ H 
+ 3-1 
■ Hygrograph curve normal. 
18 
- isi 
1 
2 
+ 7§; 
19 
+ 5 2 
+ 3 C 2 
+ 32f 
+ 32f 
Hygrometer not below 75 
)? 
20 
+ nf 
+ 
+ 49 
+ II 
from 8 p.m. on 18th till 
>> 
21 
- 3i 
+ 4 i 
~ 4 J 2 
+ 3§ , 
10 a.m. on 21st. 
22 
- 24I 
- 44l 
- 19 
-30 1 
j Hygrometer fell to 41 at 
[ 4 p.m. on 2 1 st. 
1 Summer time, i. e. one hour earlier than G.M.T. 
2 This mode of expression appears the most suitable means of indicating the amount of absorp- 
tion in relation to the total absorptive capacity. 
