50 



HEREDITY AS ILLUSTRATED BY TRICHOMES. 



Aberrant and abnormal types of trichomes were seen in both lines of 

 hybrids. These either were modifications of existing 1 trichome forms or 

 quite new types. Disk-shaped trichomes with more than 2 cells in the 

 stalk, and short secreting- trichomes with stalks with 3 in place of 2 cells 

 were observed. The first abnormality noted was seen also in the pure 

 parents, nigra and regia. The abnormal short secreting- trichome has been 

 noticed only in the hybrid Juglans calif ornica X Juglans nigra plants num- 

 bered 1 and 2. Abnormal long secreting- trichomes were seen, as above 

 indicated, but none which were certainly of the smaller form of this trichome. 



FIG. 16. Trichome of Juglans calif ornicaY. Juglans nlr/ra, F 2 , plant 4. 

 Abnormal form of type of trichome which normally has a head of 8 

 and a stalk of 8 cells. ( X 1200). 



An additional type of the abnormal trichome, the 

 irregularity of which is not dependent on the modifica- 

 tion of a form now prevalent, was observed several 

 times in Juglans californica X Juglans nigra and once 

 in the other hybrid. This trichome consists of a cell- 

 complex in which the customary supporting stalk is 

 wanting; the trichome is made up of 3 tiers of cells, of 

 which each tier has 2 (fig. 18). Thus there are 6 cells 

 in the trichome, of which 2 are terminal, 2 basal, and 2 median. This 

 trichome arises as follows: After the young organ is separated by a cross- 

 wall from the epidermis it undergoes longitudinal division (fig. 18, a), in 

 place of a transverse wall, as commonly is the case. The circumstance that 

 the first wall is longitudinal necessarily affects all subsequent cell-divisions 

 in such a manner as to bring about the origin of a new form of trichome. 

 It is of interest to note that the trichome is not a modification of an exist- 

 ing type, but that it appears suddenly and owes its existence to the unique 

 placing of the first wall. It is of interest to note further that should experi- 

 mentation show that each type of trichome is a unit character, we have 

 here the origin of a character through mutation, in place of through gradual 

 change, as may be the case with certain of the other trichomes whose 

 origin and development have been reported on in this paper. 



