FOOD SUPPLY 53 



extent outside of their natural habitat and consequently find 

 themselves frequently and to varying extents, free from at 

 least some of their original insect pests. With the gradual 

 growth of world commerce and the great increase in the ease 

 and rapidity of transportation, insects are more easily carried 

 over natural barriers and the aggregation of agricultural pests 

 is continually becoming more homogeneous. 



As was mentioned before, cultivated plants are often at- 

 tacked by insects which have previously fed on related plants 

 and such occurrences, although uncommon, are by no means 

 rare. Many of our most important food plants belong to a 

 very few of the natural families of plants and it is noticeable 

 also that these plant groups are usually very widely distrib- 

 uted ones with many representatives. Thus the grasses or 

 Graminese furnish us with wheat, Indian corn, oats, rye, 

 barley and sugar cane; the Leguminosse with beans, soy 

 beans, chick peas, peas and cowpeas; the Solanacese with the 

 potato, tomato, sweet pepper and egg plant, the Umbelliferse 

 with carrots, parsley, celery and parsnips; the Cucurbitacese 

 with the pumpkin, squash, vegetable marrow, cucumber and 

 melons; the Cruciferse with cabbage, cauliflower, Brussels 

 sprouts, kohl-rabi, mustard, turnips and radishes; the Com- 

 positse with salsify, lettuce, and the Jerusalem artichoke; the 

 Polygonacese with buckwheat and rhubarb; the Liliacese 

 with asparagus, onions, leeks, garlic, etc. 



There are, of course, very numerous exceptions to this as is 

 the case in food plants like the beet, sweet potato, etc., but 

 these only modify the general principle that the utilization 

 of numerous related plants is the common practice. It is 

 noticeable also that a good many of these plants are of 

 tropical or semitropical origin, and that they are commonly 

 cultivated in climates far more rigorous than those in which 

 they have originated. This is true of a large variety of well- 

 known food plants like corn, potatoes, tomatoes, pump- 

 kins, etc. 



As might naturally be expected from the foregoing, we 

 actually find that some of our pests of wheat are species 



