64 HEREDITY AS ILLUSTRATED BY TRICHOMES. 



From a study of the development of the trichomes \njuglans, 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 Jitg/ans 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 oijuglans 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 Jug latis, as con- 

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



Abnormal Long secreting 



type trichome in 



niyra only 



Long secreting Abnormal 

 trichome ( yp e 



^ Abnormal 



type 



Unicellular Aberrant Short secreting- 



triehome trichome trichome 



FIG. 21. ProlMilde origin and relationship of Hie trichomes of Jiiplans. 



