20 



HEREDITY AS ILLUSTRATED BY TRICHOMES. 



The disk-shaped trichomes are glandular and are to be found on both 

 surfaces of the leaves (fig'. 4, /"to ;/). 



The developmental history of the trichomes was not followed in all par- 

 ticulars. The following are some of the definite stages: 



The trichome originates as broad and squat outgrowths of an epidermal 

 cell, from which it is early separated by a cross-wall that nearly coincides 



F IG . 4. Trichomes of. Tuglans californica: a, group of awn-shaped trichomes forming a stellate 

 trichome, leaf-surface ( < 81); h to n, various stages in the development of disk-shaped tri- 

 chomes (c, d, e,h,j, and I X 535;/, g, /.and m <S60; n < 340); o, abnormal disk-shaped trichome 

 (X 535). 



with the adjacent epidermal walls. From its first appearance as a papillate 

 projection the disk-shaped trichome is easily distinguishable from the be- 

 ginnings of the other forms. The young trichome becomes 2 -celled by a 

 transverse division which separates the stalk from the portion which will 



