46 Pehr Olsson-Seffer. 
obtain in different climates and that especially local topography is 
ot great moment. The coastal sand-formations have usually an 
open vegetation for some distance from shore, when forest follows, 
and the neighbourhood of the woods affects to some extent the 
rate of evaporation. 
The influence of topography on evaporation can be noticed in 
the difference in vegetation on exposed places on the dunes, in the 
depressions and on the leeward side. Series of measurements of 
the rate of evaporation on coastal sands would be of considerable 
interest. The same objection that was expressed above regarding 
the mode of making meteorological observations in connection 
with ecological investigations holds good also in this case. The 
evaporimeters used need to be all alike, and the environmental 
conditions have to be carefully considered if the measurements are 
to be at all comparable. Relative values, which are sufficient 
for general climatological purposes are not satisfactory when we 
endeavour to interpret the ecological adaptations in the light of 
local meteorological conditions. We must attempt to get data 
representing as nearly as possible the absolute values. 
The Effects of the Sea Itself. 
The hydrodynamic factors which present themselves in the sea 
are necessarily of great moment to the adjoining shore formations, 
not only directly but also indirectly. On the submerged beach the 
seawater is the surrounding medium in which the vegetation lives, 
and all the physical properties of this medium are of the greatest 
consequence to the plants. Furthermore, wave action and currents 
in the water are here important.factors influencing the plant-life. 
On the front beach most of these factors determine the conditions 
for the vegetation, while on the formations more distant from the 
water the influence of the sea is of a more indirect nature. 
Action of Waves. There is a considerable difference in the 
influence of waves on shores with a steep, and with a gentle slope. 
The on-shore action of waves on a beach of gentle slope is of greater 
efficiency in working the sand landwards than on a steep slope. 
The undertow on the former shore is more interrupted by the 
breakers than where the shore is steep and the water deeper. 
Generally the agitation of the water on a shallow bottom is greater. 
The plants growing in places exposed to the full force of the 
on-rushing breakers must also necessarily be endowed with special 
appliances to be able to withstand the continuous wear and tear of 
the surf. Some of these adaptations will be mentioned later. 
