66 HEREDITY AS ILLUSTRATED BY TRICHOMES. 



trichomes of each pure species have a characteristic size and form, but that 

 a considerable range of variation, which is always associated in a very defi- 

 nite manner with the position occupied on the leaf, or with the age of the 

 leaf, is to be found both in form and in size. 



The first and the second generations of thejuglans hybrids were examined. 

 The first-generation hybrids of both strains, as regards the leaf -char- 

 acters, are intermediate although not strictly so. Dominance was not 

 observed. Each hybrid bears all of the trichomes found in both parents, 

 which in the nigra derivative is one more than in the other hybrid, owing 

 to the fact that pure nigra has one more type of trichome than occurs in 

 regia or calif arnica. 



So far as followed each form of trichome, type and type, had the man- 

 ner of development which was seen for it in the pure lines. In size and 

 form the trichomes are intermediate between the same characters of the 

 trichomes in the pure lines, and also in the hybrid they exhibit a varia- 

 tion directly associated with the position which they occupy on the leaf 

 or with its age. 



The second-generation hybrids of Juglans do not exhibit reversions in 

 gross leaf-characters to either pure line. The course of development of 

 trichomes in both strains was followed closely and it was learned that each 

 trichome type has the same sequence of cell-division in development as 

 was seen for the particular trichome in F, and the pure lines. The 6-celled 

 trichome has a certain form of development, which the 8-celled type fol- 

 lows exactly up to the 6-celled stage and then it adds 2 divisions peculiar 

 to itself. The other multiccllular trichomes agree with the sequence of 

 these divisions in the early stages only. 



In the hybrids of the second generation, both strains, abnormal trichomes 

 were seen. These were very evident modifications of types already exist- 

 ing. One aberrant type also was found, which was quite different from 

 any occurring commonly on the leaves. The aberrant trichome originated 

 by divisions which were essentially different from those of the usual tri- 

 chomes, and there are no intermediate forms between the aberrant trichome 

 and the other types, from which facts this type is held to be a mutation. 



(6) The trichomes of Juglans are thought to arise from three types, 

 namely, the awn-shaped, the 6-celled (short secreting), and the aberrant 

 trichomes. By the conversion of all of the epidermal cells of a group into 

 awn-shaped trichomes, stellate types arise; by the processes of arrested 

 development, or the fixation of minor variations (such as the abnormal 

 trichomes), or by mutation, the multicellular trichomes have originated. 

 In Juglans the largest number of types of trichomes seem to have arisen 

 by the second and third means. The aberrant form represents a type which, 

 by such variation, would be the ancestor of still other kinds of multicellular 



