264 SECTIONAL ADDRESSES. 
by producing a variety resistant to the disease ; or there may fortunately 
be found an immune plant from which stocks can be had, as in the case 
of the potatoes found by Mr. Gough to be immune from the terrible 
wart disease. 
Control of Environmental Factors. 
It thus appears that, if only plant breeders and plant physiologists 
could learn to alter existing plants or to build up new plants in such a way 
that they should be well adapted to existing soil and climate conditions, 
and not adapted to receive disease organisms at the time the organisms 
are ready to come—if only they could do this all agricultural land 
would become fertile and plant diseases and pests would become 
ineffective: at any rate until the pests adapted themselves to the new 
plants. Although no one can set limits to the possibilities of plant breeding 
and plant physiology, we cannot assume that we are anywhere near this 
desirable achievement or that we are likely to be in our time. There will 
always remain the necessity for altering the environmental conditions to 
bring them closer to the optimum conditions for the growth of the plant. 
No attempt is yet made in the field to control two of the most important 
of the factors: the light and the temperature, though it is being tried 
experimentally. There is a great field for future workers here ; at present 
plants utilise only a fraction of the radiant energy they receive. At 
Rothamsted attempts have been made by F. G. Gregory to measure this 
fraction ; the difficulties are considerable, but the evidence shows that our 
most efficient plants lag far behind our worst motor-cars when regarded 
as energy transformers for human purposes. One hundred years ago the 
efficiency of an engine as transformer of energy was about 2 per cent. ; 
now, as a result of scientific developments, it is more than 50 per cent. 
To-day the efficiency of the best field crops in England as transformers of 
the sun’s energy is about 1 per cent.1: can we hope for a similar develop- 
ment in the next hundred years ? If such an increase could be obtained 
an ordinary crop of wheat would be about 400 bushels per acre, and farmers 
would feel sorry for themselves if they obtained only 200 bushels. But we 
are only at the beginning of the subject. Increases in plant growth amount- 
ing to some 20 or 25 per cent. have been obtained by V. H. Blackman in 
England under the influence of the high-tension electric discharge, which 
presumably acts by increasing in some way the efficiency of the plant as an 
energy transformer. Possibly other ways could be found. It needs only a 
small change in efficiency to produce a large increase in yield. Much could 
be learned from a study of the mass of data which could be accumulated 
if agricultural investigators would express their results in energy units as 
well as in crop yields as at present. 
Interesting results may be expected from the attempts now being made 
in glasshouse culture both in Germany and at Cheshunt to increase the 
rate of plant growth by increasing the concentration of the carbon dioxide 
in the atmosphere. 
Control of the Soil Factors. 
The soil factors lend themselves more readily to control and much has 
been already achieved. Water supply was one of the first to be dealt 
1 The remaining energy being largely used up in transpiration. This figure refers 
to the total radiation received by the leaf, and not to the fraction received by the 
chloroplast surface. For this latter the value is much higher. 
