PRESIDENTIAL ADDRESS. 641 
from growth, though hardly in the condition of looseness that could be 
described as a soil mulch. Yet the evaporation, even under a moist English 
atmosphere, amounts to one-half of the annual rainfall, and the significant 
thing is that the evaporation is approximately the same from all of the 
gauges and is independent of the depth of subsoil within which water is 
stored. Evaporation, then, would seem to be determined by surface alone, but 
we are without systematic experiments to show how variations in the surface 
induced by cultivation will alter the rate of evaporation. A knowledge 
of the evaporation factor would then inform us of what proportion of the 
rainfall reaches the subsoil; we then want to know to what extent it can 
be recovered and how far it may sink beyond the reach of the crop. It is 
commonly supposed that the subsoil below the actual range of the roots of 
the crop may still return water by capillarity to the higher levels that are 
being depleted, the deeper subsoil thus acting as a kind of regulating reservoir 
absorbing rain in times of excess and returning it when the need arises. But 
some work of Leather’s in India and Alway’s on the great plains of North 
America throw doubt on this view, and would suggest that only the layer 
traversed by roots, say, down to a depth of 6 feet, can supply water to the 
crop; the water movements from the deeper layers due to capillarity being too 
slow to be of much effect in the maintenance of the plant. The evidence on 
either side is far from being conclusive and more experiment is very desirable. 
It would also be valuable to know how far evaporation from the bare 
soil can be checked by suitable screens or hedges that will break the sweep 
of the wind across the land. In England hedges have always been looked 
at from the point of view of shelter for stock; we find them most developed 
in the grazing districts of the west, while bare open fields prevail in the east 
and south. Yet the enormous value of a wind-screen to vegetation can be 
readily observed, and the market-gardeners both in England and the still 
dryer districts of the south of France make great use of them. Lastly, we 
must have more knowledge about the relation between transpiration-water and 
growth : we do not know if the high ratios we have spoken of hold for all 
plants. Xerophytic plants are supposed to be possessed of protective devices 
to reduce loss of water. Are they merely effective in preserving the plant from 
destruction during the fierce insolation and drying it receives? and do they 
enable a plant’to make more growth on a given amount of water? Wheat, for 
example, puts on its glaucous waxy bloom under dry conditions. Is this really 
accompanied by a lower rate of transpiration per unit surface of leaf? and is it 
more than defensive, connoting a better utilisation of the water the plant 
evaporates ? 
The cultivation of these soils with a minimum rainfall necessitates varieties 
of plants making a large ratio of dry matter to water transpired and also with 
a high ratio between the useful and non-useful parts of the plant. Mr. 
Beaven has shown that the difference in the yields of various barleys under 
similar conditions in England are due to differences in their migration factors : 
the same amount of dry matter is produced by all, but some will convert 50 
per cent. and others only 45 per cent, into grain. This migration ratio, as 
may be seen by the relation between corn and straw on the plots at Rothamsted, 
is greatly affected by season; nevertheless Mr. Beaven’s work indicates that 
under parallel conditions it is a congenital characteristic of the variety and 
therefore one that can be raised by the efforts of the plant-breeder. The needs 
of dry-land-farming call for special attention on the part of the breeder to these 
two ratios of transpiration and migration. 
Closely linked up with the problems of dry-land-farming are those which 
arise in arid climates from the use of irrigation-water on land which 
is either impregnated with alkaline salts to begin with or develops such a con- 
dition after irrigation has been practised for some time. The history of 
irrigation-farming is full of disappointments due to the rise of salts from the 
subsoil and the subsequent sterility of the land, but the conditions are fully 
understood and there is no longer any excuse for the disasters which have 
overtaken the pioneers of irrigation in almost every country. Sterility may 
arise from two causes—overmuch water which brings the water-table so close 
to the surface that the plants’ roots may be asphyxiated, or the accumulation 
1914. TT 
