[单项选择]

Diseases of Agricultural Plants
Plants, like animals, are subject to diseases of various kinds. It has been estimated that some 30,000 different diseases attack our economic plants; forty are known to attack com, and about as many attack wheat. The results of unchecked plant disease are all too obvious in countries which have marginal food supplies. The problem will soon be more widespread as the population of the world increases at its frightening rate. Even in countries which are now amply fed by their agricultural products there could soon be critical food shortages. It is easy to imagine the consequences of some disastrous attack on one of the major crops; the resulting famines could kill millions of people, and the resulting hardship on other millions could cause political upheavals disastrous to the order of the world.
Some plants have relative immunity to a great many diseases, while others have a susceptibility to them. The tolerance of a particular
A. Around 30,000.
B. Around 140.
C. Around 29,960.
D. Around 40.

[单项选择]

Diseases of Agricultural Plants
Plants, like animals, are subject to diseases of various kinds. It has been estimated that some 30,000 different diseases attack our economic plants; forty are known to attack com, and about as many attack wheat. The results of unchecked plant disease are all too obvious in countries which have marginal food supplies. The problem will soon be more widespread as the population of the world increases at its frightening rate. Even in countries which are now amply fed by their agricultural products there could soon be critical food shortages. It is easy to imagine the consequences of some disastrous attack on one of the major crops; the resulting famines could kill millions of people, and the resulting hardship on other millions could cause political upheavals disastrous to the order of the world.
Some plants have relative immunity to a great many diseases, while others have a susceptibility to them. The tolerance of a particular
A. Some plant diseases may be caused by bacteria.
B. Some plant diseases may be caused by pathogens.
C. Some plant diseases may be caused by fungus.
D. Symptoms are always helpful in identifying diseases.

[单项选择] Diseases of Agricultural Plants
Plants, like animals, are subject to diseases of various kinds. It has been estimated that some 30,000 different diseases attack our economic plants; forty are known to attack com, and about as many attack wheat. The results of unchecked plant disease are all too obvious in countries which have marginal food supplies. The problem will soon be more widespread as the population of the world increases at its frightening rate. Even in countries which are now amply fed by their agricultural products there could soon be critical food shortages. It is easy to imagine the consequences of some disastrous attack on one of the major crops; the resulting famines could kill millions of people, and the resulting hardship on other millions could cause political upheavals disastrous to the order of the world.
Some plants have relative immunity to a great many diseases, while others have a susceptibility to them. The tolerance of a part
A. A world war.
B. Border conflicts.
C. Rations of grain and meat.
D. Social upheavals.

[简答题] What is a Species A species of plants or animals, as the term has been generally used by naturalists, comprises all such individuals as are so similar to each other that we may suppose them all to have proceeded from one common parentage, and so dissimilar from all others that they could not have been reproduced from the others, nor the others produced from them. All we know is, that the plants and animals throughout the world exist in species, each one of which stands at present distinct and isolated wholly apart from all the rest, and one cannot be transformed into another by ordinary generation, through changes of soil and climate, or any other causes whatever known to man, within so short a period as six thousand years. The apple, for instance, is one species, and the pear is another. In many respects they are similar to each other, and each may be changed by cultivation and by the operation of other causes a great deal, but by no po