64 



HEREDITY AS ILLUSTRATED BY TRICHOMES. 



From a study of the development of the trichomes in Juglans, as just 

 summarized, it would appear that there may be at least two ways they 

 may take their origin. They either may be modifications of types already 

 existing-, as illustrated by the abnormal forms and indicated by a com- 

 parison of the development of the trichomes, or they may arise suddenly 

 through the circumstance that the initial cell-division is a unique one. In 

 addition to the origination of the trichomes, in the manners described, 

 observations suggested that at least compound unicellular trichomes might 

 arise in quite another manner. 



In most of the Juglans studied the awn-shaped trichomes occur singly, 

 but in a few instances they were aggregated into small groups, the elements 

 of which, however, were quite the same as the single trichomes. In such 

 cases it is evident that the more complex trichomes, referred to in fore- 

 going descriptions as stellate trichomes, owe their origin to the fact that 

 several adjacent epidermal cells all give rise to awn-shaped types, and 

 that therefore they can not become separated in the subsequent develop- 

 ment of the leaf. The primitive types of trichomes oi Juglans may, there- 

 fore, be conceived of as being the awn-shaped trichome, in addition to the 

 short secreting trichome and the aberrant type. The possible relationships 

 of these types, and their relation to the other trichome in Juglans, as con- 

 ceived from their study, is graphically shown in fig. 21. 



Abnormal 

 type 



IftiieeJlular 

 frichame 



Aberrant 

 trichome 



Long secreting 

 trichome in 

 nigra only 



t 



Long secreting 

 trichome ~ 



\ 



Short secreting- 

 trichome 



Abnormal 

 type 



Abnormal 

 type 



Fig. 21.— Probable origin and relationship ot the trichomes of Juglans. 



