10 TOBACCO BREEDING. 



crop not only results in a lov\^ yield, as a whole, and more especially 

 of the best and most profitable grades of the cured and fermented 

 product, but also increases the cost of sorting out the different 

 types of leaves into their respective grades for market, the expense 

 of which must be borne directly or indirectly by the grower. 



The principal cause of the lack of uniformity in tobacco is cross- 

 fertilization. In tobacco, as in all other crops, seed resulting from 

 cross-fertilization produces many plants unlike either parent. There- 

 fore such seed is undesirable for the general planting of a crop 

 where uniformity is so important a factor, "^^liere the tobacco seed 

 plants are grown without protection from cross-fertilization some 

 of the flowers are cross-fertilized by insects or other agencies. De- 

 sirable plants may thus be crossed with undesirable plants in the 

 same field or in adjoining fields, and the plants grown from the seed 

 thus produced are usually extremely variable, some of them resem- 

 blino- the desirable plants from which the seed was harvested, others 

 resembling the inferior plants from which the pollen was carried 

 for crossing, while the remainder are of an intermediate type, un- 

 suited to the purpose for which the crop is grown, and therefore 

 causing a loss to the grower. The writers have observed number- 

 less cases in different tobacco-growing sections where several dis- 

 tinct and worthless new types appeared m the fields, the plants of 

 which were grown from carefully selected seed. These undesirable 

 types could only be accounted for by the accidental crossing of the 

 seed plants the year preceding or at some previous time. The cross- 

 ing of individual plants of the same strain, even if both are desirable 

 plants, results in undesirable variations, many of which are appar- 

 ently reversions to earlier and unimproved types of tobacco. 



In those varieties of tobacco in which the buds are removed long 

 before the flowers open on all of the plants except those saved for 

 seed production, or where early topping is practised, the opportunity 

 for the crossing of the flowers borne by the seed plants with other 

 plants in the same field is almost wholly limited to the seed plants. 

 However, it frequently occurs that late or diseased plants, or pos- 

 sibly sucker branches that have been overlookt, develop flowers 

 which open at just the right time to allow insects to cany the pollen 

 from these flowers to the seed plants and thus effect cross-fertiliza- 

 tion. There is little doubt that many of the plants of irregular and 

 unusual types are produced as a result of this kind of cross-fertiliza- 

 tion. 



An important cause of variation in tobacco plants is the use of 

 immature seed. Many growers cut off or harvest the seed heads 

 before all of the seed pods have turned brown; hence, before ma- 

 turity. The writers have observed hundreds of instances where the 



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