Marsh, and its Relations to Evaporation, &c. 
3^9 
Yapp, R. H. (’07) : On the Hairiness of certain Marsh Plants. Report of the British Association, 
Leicester Meeting, 1907, p. 691. London, 1908. 
(’08) : Sketches of Vegetation at Home and Abroad. IV. Wicken Fen. New 
Phytologist, vol. vii, p. 61, 1908. 
EXPLANATION OF FIGURES IN PLATE XX, AND TEXT- 
FIGURES 5, 7 and 8. 
Illustrating Professor Yapp’s paper on the Vegetation of a Marsh. 
PLATE XX. 
Fig. 1. ‘Sedge ’ vegetation, about four feet high. Cladium Mariscus is dominant, but Phrag- 
mites communis , Thalictrum flavum, Hydrocotyle vulgaris , Valeriana officinalis, Lysimachia 
vulgaris , &c., are present. 
This photograph was taken in July, 1907, near the stand of instruments. Note the density 
of the vegetation, and the ‘ general vegetation level ’. Only the tips of the leaves of Cladium , and 
a few shoots of Phragmites , &c., project above this general level. During a wind, the tips of the 
leaves are beaten down, and the general vegetation level then becomes remarkably uniform 
(cf. Text-fig. 4). 
Fig. 2. ‘Litter’ vegetation (about two feet high), in which the 1908 experiments were 
conducted. Note the A and B sets of instruments. C is hidden in the lower strata of the vegeta- 
tion. Carices are the most abundant plants in this society, but many other species are freely inter- 
mingled with them. 
TEXT-FIGURES 5, 7 and 8. 
Text-Figs. 7 and 8. Curves of evaporation and temperature for the period July 31 
to August 9, inclusive. 
The curves are plotted with intervals of time as abscissae ; and as ordinates either (1 ) the rates 
of evaporation in c.c. per hour, or (2) temperature in degrees centigrade. 
Readings were taken regularly every two hours from 8 a.m. to 8 p.m. throughout the period. 
The only exception to this was on July 31, when the first reading was at 10 a.m. There was also 
an additional reading at 6 a.m. on August 1. The times of observation are indicated by the short 
vertical lines below the numbered hours. 
The letters A, B and C refer to the respective positions of the three sets of instruments 
(see Text-fig. 6). 
(a) Curves of Evaporation. These are constructed by taking the average rate of evaporation 
per hour for the period between two observations, and plotting this in the middle of the period. The 
same method is employed for both day and night. The night periods, however, were twelve hours 
in length, as compared with the two-hour periods during the day, and the curves are therefore pro- 
portionately less accurate. The true curves of night evaporation, especially for the A position, 
should be somewhat flatter than represented here. 
( b ) Curves of Temperature. So far as the day periods are concerned, the curves are constructed 
in the same way as those of evaporation. The average temperature for the period is found by taking 
the mean between the maximum and minimum readings. For the night periods, however, the means 
would be too high, owing to the usually great rise of temperature shortly before 8 a.m. The abso- 
lute minimum for the night was therefore taken, and plotted midway between 8 p.m. and 8 a.m. 
This double method gives curves which represent the actual variations of temperature more nearly 
than if the means had also been used for the long night periods. The difference in method is 
indicated by a break in the continuity of the curves between day and night. 
The curves forming Text-fig. 5 are constructed in the same way, except that soil temperatures 
are added. In the case of the latter, the diurnal range is so small, that the absolute maxima and 
minima (and not the means) are used for both day and night. 
