ALBERT AND GABRIELLE HOWARD. 65 



thus showing to what a great extent natural crossing is possible 

 when the various kinds are grown in close proximity and allowed 

 to flower freely. Some of the aberrant plants only differed very 

 slightly from the rest of the culture and much care was necessary 

 to distinguish them. Sometimes they appeared onty a little 

 more robust than the rest, but in the case of every plant which 

 showed the slightest variation in 1908, its progeny, after bagging, 

 gave rise to uneven cultures made up of many different types. 



Too much emphasis cannot be laid on the difficulty of 

 observinof the first greneration of a cross between two kinds 

 of tobacco in the field. The differences between many tobacco 

 types are exceedingly small aijd can only be detected with 

 precision after considerable study of such types when grown side 

 by side and allowed to flower. Crosses between types, unless 

 they are very different, would not be easy to observe in the 

 Fj generation even in cultures from single plants. In the 

 field, w^here the great majority of the plants are topped, these 

 differences would, in all probability, be overlooked. In the second 

 and succeeding generations, when splitting takes place, the various 

 types resulting from crossing could for the first time be observed. 



In great contrast to the variable character of the progeny 

 of a selfed aberrant plant is the great uniformit}^ of the produce 

 obtained from the seed of an ordinary plant raised under bag. In 

 Plate II some of these constant pure line cultures are shown. 

 These cultures afford a means of observing the occurrence or 

 otherwise of mutations and also of the amount of variation that 

 occurs in the types of tobacco. No mutations have been 

 observed and every case of variation from the type has so far 

 been proved to be due to natural crossing. These cultures show 

 the remarkable uniformity of the plants of any particular type 

 both in the same year and also from year to year. 



From the practical point of view the significance of natural 

 cross-fertilization is very great. The desirable types of tobacco 

 grown in any district or introduced from other places must be .self- 

 fertilized if the quality of the type is to be maintained. The seed 

 sown must have been raised under bao^. Where a large number of 



