40 



HEREDITY AS ItLUSTRATED BY TRICHOMES. 



The average length of the heads of trichomas of old leaves is 20.6 M; 

 the average diameter is 25.5 M. Thus the lengths of heads in the old leaf 

 and in the young leaf are practically the same. In the pure species, where 

 the comparison is made, the young leaves were found to have the largest 

 trichomes. It is possible that a comparison of trichomes from analogous 

 leaf-surfaces would in this instance also give similar results. 



Trichomes of Juglans californica X Juglans regia, Fj. 



As in the pure species Juglans californica and Juglans regia, so also in 

 the hybrid, 4 types of trichomes are to be distinguished, namely, the awn- 

 shaped, the disk-shaped, and the long and the short secreting trichomes. 

 The unusual type of long secreting trichome which was observed in the 

 other hybrid Juglans and in the pure species Juglans nigra appears not 

 to be present in this hybrid (fig. 10). 



Table 17. — Measurements on heads, of long secreting trichomes of 

 Juglans californica X Juglans regia, F^. 



The awn-shaped trichomes are most abundant in the young leaves, where 

 they occur on the ventral surface only. They are of the usual type, uni- 

 cellular, and range around 211.6 /"■ in length. 



The disk-shaped trichomes, which in structure are quite like those in 

 both pure lines, consist of a flattened, multicellular head borne on a stalk 

 of 2 cells. In young leaves the trichomes are to be found on both sur- 

 faces; in older ones they occur on the ventral surface only. Mature tri- 

 chomes of this type vary greatly in diameter, but, so far as observed, the 

 differences in diameter are more or less closely associated with differences 

 in position on the leaves, and possibly also with the age of the leaf. The 

 following measurements, in /^, give the diameters of representative disk- 

 shaped trichomes: 71.4, 73.6, 67.2, 65.0, 75.6, 92.2, 79.8. Of these the 

 two last were from the region of the veins. The average diameter of the 

 heads of the trichomes is 74.9 /^. A few measurements were made also to 

 trace an additional possible character— the depth of the depression— for 

 comparison with the pure lines . These measurements are as follows, in /*: 

 diameter, 63.0, 84.0, 92.4, 79.8; depth, 12.6, 16.8, 12.6, 12.6, respectively. 



The long secreting trichomes are most abundant on young leaves and 

 are more abundant on the veins than between them. They occur on both 

 leaf -surf aces. The trichome studied consists of a head with 4 radiate 

 cells and a stalk of 4 cells. None of the type with a head of 8 cells was 



