30 



HEREDITY AS ILLUSTRATED BY TRICHOMES. 



On the veins the average length of the heads is 20.8 A*; the average 

 diameter is 31.5 p-. Between the veins the average length of the heads is 

 16.2 A 4 ; the diameters are not given. The variation from the average 

 length of the heads of trichomes from the region of the veins is 31.5 per 

 cent; the variation in diameter is 60 per cent. The variation in length of 

 heads from trichomes which were from between the veins is 0.9 per cent. 



TABLE 11. Measurements of short secreting trichomes from old leaves, 

 Jiiglans nigra. 



*Vein. 



The average length of heads from the dorsal surface is 16.8 A*; the aver- 

 age length of heads of trichomes from the ventral surface, including the 

 abnormally large one, is 13.4 A*; omitting that one, the length is 12.5 A*. 

 The average length of the trichomes on the dorsal surface is 35.0 A 4 ; the 

 average length on the ventral surface is 26.1 A 4 - The variation in length 

 of the heads from the dorsal surface is 20 per cent; from the ventral sur- 

 face 62 per cent. 



From the study of the short secreting trichome it appears, therefore, 

 that trichomes from the regions of the veins in the young leaves have 

 longer heads than those from the areas between them, and that the length 

 of the heads of trichomes from the dorsal surface of old leaves is greater 

 than from the ventral surface. The entire trichomes also are longer on 

 the dorsal surface; and the heads of trichomes of young leaves are greater 

 than the length in old leaves. 



JUGLANS REGIA. 



The types of trichomes which were seen in the two species of Jug Ian s as 

 above described were observed in Jug lam regia also. These are, (l) the 

 awn-shaped trichome; (2) the disk-shaped trichome; (3) one or perhaps 

 two types of long secreting trichomes, and (4) the short secreting tri- 

 chomes (fig. 8). 



