58 



HEREDITY AS ILLUSTRATED BY TRICHOMES. 



Four types of trichomes were seen in the pure species californica, nigra, 

 and regia, and in the hybrids Jnglans californica X Juglans nigra and 

 Juglans californica X Juglans regia, but in the species nigra and the hybrid 

 into which it enters an additional form of trichome, not observed in the 

 other plants, was also seen. In addition to these, the regular types, there 

 were also abnormal forms of trichomes which were clearly modifications 

 of those more commonly present, and one type which was essentially 

 different from those most abundant. With one exception all types of 

 trichomes were multicellular and glandular. 



In the second generation of the hybrid Juglans californica X Juglans nigra, 

 two of the multicellular trichomes, namely, the short and the long secret- 

 ing trichomes, are composed of 6 and 8 cells, 

 respectively. The sequence of cell-divisions 

 up to the formation of 6 cells is identical, 

 hence the larger trichome may be regarded 

 as a further developed short secreting one, 

 or vice versa, the smaller type may be held to 

 represent an arrested stage in the develop- 

 ment of the long secreting trichome (fig. 20). 

 It does not seem probable that both have 

 descended from a common ancestor (so to 

 speak) which had intermediate characters, 

 since the simplicity of the structure seems 

 to preclude this. Of the two other types of 

 multicellular trichomes which occur in the hybrid, and which have from 

 16 to 32 or more cells, the disk-shaped trichome agrees in the sequence of 

 its divisions with the two trichomes mentioned above in the first two 

 divisions only, and the fourth type of multicellular trichome agrees in 

 sequence with the two trichomes mentioned in the first 3 and possibly the 

 first 5 cell-divisions. 



The sequence of all of the cell-divisions in the 6-celled and the 8-celled 

 trichomes was found, by repeated observations, to be perfectly consistent, 

 and the first two or first three divisions also in the larger trichomes were 

 seen to be consistent for the particular type of trichome, but the subsequent 

 cell-divisions of the larger trichomes appeared not to occur in consistent 

 sequence. The last statement is made with reservation, however, as 

 further and careful study of later stages, which would be a difficult task, 

 might reveal an unexpected regularity of cell -division in such trichomes. 



The abnormal trichomes were of the following types: they were short 

 secreting trichomes with one additional stalk-cell, or a disk-shaped tri- 

 chome with 1 or 2 additional cells in the stalk. There seemed also to be 

 abnormal forms of the long secreting trichome in which the stalk had an 

 unusually large number of cells and the head also had a larger number of 



Fi. 20. Sequence in cell-division 



in the short and the long secreting 

 trichomes of Juglunx. The num- 

 bers give the succession of cell- 

 will formation in proper sequence. 



