CEREAL ADAPTATION AND ASSOCIATION 299 



and India, certain spring varieties (especially durum 

 wheats) are grown because of the brief rainfall period, 

 or because of their comparative resistance to attacks of 

 fungi and insects. Winter hardiness is sometimes not a 

 varietal distinction, even where winters are severe. For 

 instance, the Odessa or Grass wheat, formerly grown in 

 Kansas and Nebraska, succeeded well whether sown in 

 fall or spring. Rustproof oats and Beldi barley are 

 examples of southern winter varieties sown in the spring 

 in the North. 



In almost all cases it is possible, however, to identify 

 a true spring or winter variety, as such, in the seedling 

 stage in the field. If any cereal, after spring sowing, 

 makes a small leaf growth and does not at once grow erect, 

 but spreads out (in apparent anticipation of winter), it is 

 certainly a winter variety and will not mature that season. 

 This spreading habit of growth is the most essential gross 

 distinction separating spring and winter cereals. The 

 opposite tendency of a true spring variety to grow erect 

 if sown in the fall is not quite so dependable, but can 

 usually be trusted in connection with the two other char- 

 acters of color and size of leaf. There may be other 

 characters distinguishing winter and spring varieties yet 

 to be determined. 



325. Effect of numbers on the food supply. An im- 

 portant effect of crowding in a plant community is a 

 more rapid exhaustion of the food supply than in cases 

 where individuals of the same variety are isolated. If a 

 second crop of the same variety is grown the following 

 season on the same land, the exhaustion of essential foods 

 (which may have existed in comparatively small amounts 

 at first) becomes still greater (331). It may soon be 

 necessary either to increase the store of these foods by 



