44 HEREDITY AS ILLUSTRATED BY TRICHOMES. 



the leaves of both series does not indicate distinct reversion to pure lines 

 in any instance. In Juglans calif arnica X Juglans nigra, in plant No. 1, 

 the leaflets numbered from 13 to 18; in No. 4, from 10 to 11. In the pure 

 line calif arnica the leaflets numbered from 19 to 21, and in nigra from 13 

 to 21. It therefore appears that the hybrid No. 4, which is small in size, 

 has a smaller number of leaflets than were seen in any leaf of either pure 

 line. \T\Juglans calif arnica X Juglans regia, on the other hand, in num- 

 ber the leaflets are intermediate, since in A they range from 7 to 11, and 

 in C from 7 to 9, while in regia the range is from 5 to 7. It is, therefore, 

 concluded, as regards the plants which were especially studied of both 

 series, that they do not show reversion in any gross leaf-character to pure 

 lines in any instance . 



TRICHOMES OF THE HYBRIDS. 



The material for study was found to be especially favorable for the study 

 of the embryogeny of the trichomes, and, accordingly, an account will be 

 given here of the origin and the development of the leading types. It 

 should be stated at the outset that the cell-divisions were seen in numbers 

 sufficiently large to point to the soundness of the conclusions based on 

 this phase of the investigation. 



Five forms of trichomes were found in the hybrid Juglans calif arnica X 

 Juglans nigra and 4 in the other hybrid. The type not common to both 

 is the long secreting trichome already noted as occurring in Jiiglans nigra 

 and in the first-generation hybrid with nigra blood. In addition to these 

 trichomes, which do not require any additional description here, 3 or 4 

 abnormal types were seen, all but one of which had already been noted. 

 These will be described below. Since the development of any type of tri- 

 chome adheres to its peculiar pattern in whichever strain it is found, unless 

 especially stated to the contrary, the subjoined descriptions apply to both 

 lines. 



The disk-shaped trichome takes its origin as a squat projection of an 

 epidermal cell and early undergoes transverse division. The first division 

 of the unicellular trichome thus formed is a transverse one by which the 

 portion which is to become the head is separated from the portion to 

 become the supporting stalk. The second division is a longitudinal one in 

 the end-cell. This sequence was observed without exception in the disk- 

 shaped trichome, and is the sequence of the first two cell-divisions in all 

 of the other multicellular trichomes, save only a single aberrant type which 

 will be mentioned below. In all cases examined the next divisions occur 

 in the head, which appears to become 4-celled at least prior to the trans- 

 verse cell-division which completes the divisions of the stalk ; and tri- 

 chomes were observed with the fifth and sixth head-cells forming, and one 

 with ahead of 8 cells without the final division of the stalk-cell. On the 

 other hand, trichomes were seen which had the stalk fully developed, 



