toxins or the acquiring: for plants of immunity through the pro- 
duction of antitoxins or the use of drugs is still extremely meagre. 
By far the greatest number of plant diseases are caused by 
fungi, well known representatives of which are mushrooms and 
toadstools, yeasts and molds on bread. Some have quite con- 
spicuous spore-bearing bodies, such as the shelf-fungi, which 
sometimes form on trees; in others the spore-bearing parts 
are more or less obscure, as in the rusts of wheat and the chest- 
nut blight fungus. In all of them the fungus threads bore down 
into the host plant and rob it of nourishment; and the disease is 
scattered from one plant to another by means of minute spores. 
A comparative few of the diseases of plants are caused by 
seed-bearing parasites. Among those most commonly met with 
are mistletoe, parasitic on the branches of various kinds of trees 
in the South and West; dodder, sometimes a serious disease in 
clover, alfalfa and flax fields; beech-drops, broom-rape and 
cancer-root. 
The plant doctor has been entirely successful in combatting 
many of these plant diseases; while in the case of others, such as 
the chestnut blight, he has been unable so far to check the spread 
of the disease. While much progress has undoubtedly been made 
in our knowledge as to the life histories of plant diseases and as 
to how to prevent or cure them, plant pathologists agree that 
their science is as yet in its infancy, and that the field of unex- 
plored knowledge is vast. We have learned this much, however, 
that our main methods of attack against most plant diseases 
should look to prevention. It is true that by providing better 
sanitation, or by the careful spraying of plants, even after the 
disease has shown itself, we can sometimes control disease by 
preventing its spread, yet we must acknowledge that even in these 
instances prevention is the best cure. 
The Plant Quarantine act of 1912, administered by the Federal 
Horticultural Board of the United States Department of Agricul- 
ture, is especially designed to prevent by controlling importations 
such deplorable accidents as that which permitted the importa- 
tion of the chestnut blight fungus from China. Besides trying 
thus to keep dangerous pests out of the country, plant patholo- 
gists are also attempting by means of breeding and selection to 
discover those plants which are resistant to diseases and thus 
help from another angle to solve the problem of the control of 
plant diseases. It appears to some, for example, that the only 
feasible method of attack of the chestnut blight is to find some- 
where in the world, perhaps in China, where the disease is 
thought to have had its origin, chestnut trees which resist the 
disease to which our native trees apparently so readily succumb. 
It at least indicates much promise of success along these lines to 
