34 HEREDITY AS ILLUSTRATED BY TRICHOMES. 



GENERAL COMPARISON OF THE LEAVES AND THE TRICHOMES 

 IN PURE SPECIES OF JUGLANS. 



In leaf-characters the pure species of Juglans are fairly sharply separated 

 from one another. For example, in number of leaflets regia has less than 

 one-half those of either of the other species, and the range in number of 

 leaflets in calif ornica is considerably less than the range in nigra. In form 

 of the leaflets the species are not so sharply distinguishable. The leaf- 

 character, however, is a good one to observe in the hybrids between these 

 species. 



Although somewhat difficult in all cases to speak with certainty, with a 

 single exception it appears that all of the types of trichomes occurring in 

 any one species may be found in all, and, further, that the leading points 

 in structure and development are the same wherever the trichome is to be 

 found. Four types of trichomes occur in calif ornica and regia and an ad- 

 ditional type in nigra. Of these, one form is unicellular and lifeless, the 

 others are multicellular and glandular. So far as could be determined 

 from the material at hand, each type of multicellular trichome appears to 

 have its peculiar manner of development, as suggested above, to which it 

 adheres with great consistency. After the trichome rudiment has been 

 formed the divisions of the short secreting trichome were, first, a cross- 

 wall separating the head from the stalk; second, a longitudinal wall in the 

 head; third, a cross- wall in the stalk; and fourth and fifth, two longitudinal 

 divisions of the head. The long secreting trichome experiences the same 

 sequence of cell-divisions and to them adds 2 in the stalk. The sequence 

 of division of the disk-shaped trichome is the same for the first two divisions, 

 after which it divides in another manner, which is thought to be, although 

 not actually proved to be, also consistent. As a result of the divisions the 

 heads of the various trichomes become 4, 8, and probably 32 celled. 



Although abnormalities were observed they were not always intermediate 

 between any of the types, and in fact no intermediates were noticed. 



The multicellular trichomes varied in size to a considerable degree, but 

 the variation was fairly consistent and was usually to be related to position 

 on the leaf or to age of the leaf. The trichomes and the heads of trichomes 

 also were each longer on the dorsal surface than on the ventral, on old 

 than on young leaves, and on the veins than between them. Nothing was 

 observed on the changes in size of the trichome-heads accompanying their 

 functional activity, if such occurs, and the variation is probably to be ex- 

 plained on other grounds. 



The heads of the trichomes in the pure species Juglans californica are 

 markedly longer than those of either of the other species; those of Juglans 

 nigra are shortest. The differences in length lie from 50 to 75 per cent, 

 so that this is a very good character to study in any cross in which Juglans 

 californica is one of the parents. 



