H. M. Leake 225 



determination of the value e where an accessory notch {vide Fig. 1, 

 p. 220) occurs in the re-entrant angle at the base of the main lobe. 

 Such precautions are obviously necessary and cannot affect the value 

 of the leaf factor as a definite character. 



The determination of the leaf factor for many thousands of plants 

 has brought one remarkable feature into prominence. While every 

 value has been obtained for the leaf factor from 1 ("broad" lobed) 

 to 5 ("narrow" lobed) no case has been observed in which a plant 

 with intermediate value (between 2 and 3) for the leaf factor breeds 

 true to this character. All pure plants, and consequently all types, are 

 divisible into two distinct groups : 



(1) With a leaf factor less than 2. 



(2) „ „ gi-eater than 3. 



Within the limits 1 to 2 occur all the "broad" lobed types, while 

 ■within the limits 3 to 5 occur all the "narrow" lobed types\ 



The accuracy of the expression — that is, the measure of agreement 

 between different individuals of one type — is such that it is possible 

 to recognise within, and isolate from, a type, otherwise pure, races 

 separable only by the leaf factor. It seems probable that the existence 

 of such "pure lines," to use Johannsou's term (10), is a phenomenon of 

 general occurrence throughout this series of Gossypia and in some of 

 the types such forms have been isolated. Thus within type 4 occur 

 three "pure lines" with leaf factors of 1-37, 1-46 and 1-64 which have 

 been isolated, and from type 9 " pure linos " with leaf factors of 3'34 

 and 3-59 which have similarly been isolated. Opportunity has not been 

 forthcoming for treating this question in the detail it deserves and it 

 seems probable that with a more detailed examination the number 

 might be considerably increased. Indications of the existence of such 

 " pure lines " are apparent in Table X. 



The behaviour of the leaf factor when crossing occurs. 



When a plant with leaf factor less than 2 is crossed with a plant 

 with the leaf factor greater than 8 the leaf factor of the plants of the 

 F^ generation is found to approximate to the mean of the two parental 

 leaf factors. Table XI illustrates this point. At the time the crosses 

 were made the character had not been identified and the figures given 



1 In the fields plants are frequently founil with a leaf factor less than 3 and greater 

 than 2, and on this fact among others the author has based his views on the occurrence 

 of cross-fertilisation under natural conditions (11). 



