HYBRIDS OF JUGLANS. 



57 



The averages of the measurements are as follows: Dorsal surface be- 

 tween veins, entire length 33.5 p-, length of head 17.6 p-, diameter of head 

 27.4 p-; on veins, entire length 37.69 p-, length of head 19 p-, diameter of 

 head 26 p-; ventral surface, between veins, entire length 27. 6 p-, length of 

 head 15.5 P-, diameter of head 26 p-; on veins, entire length 35.2 p-, length 

 of head 18.9 /, diameter of head 28.3 p-. 



From the averages it appears that the trichomes which occur on the 

 veins are larger than those between them, and that those on the dorsal 



surface are larger than those 

 on the ventral . There is very 

 little difference between the 

 measurements of trichomes of 

 plant A, which is among the 

 largest of those of the second 

 generation, and of plant C, 

 which is of the smallest of this 

 generation. The difference is 

 so slight that the sum of the 

 averages of all dimensions of 

 the trichomes of A are only 5 

 more than the sum for C . The 

 form of the head is the same 

 as in A. 



In hybrid numbered A, so in C, the fact of reversion to the pure lines 

 is not clear. The heads of the trichomes of the hybrid are broader as well 

 as shorter than those of regia, but they are smaller in these dimensions 

 than the heads of calif ornica. This condition is shown diagrammatically 

 by fig. 19, which is based on the average measurements of the two pure 

 lines and the hybrid. It will be seen that, in form at least, the heads of 

 the trichomes of the hybrid agree very well with the pure species regia, to 

 which they appear to revert. 



GENERAL COMPARISON OF TRICHOMES IN THE PURE SPECIES 

 AND HYBRIDS OF JUGLANS. 



The study of the trichomes of the Juglans hybrids and pure species has 

 been carried on with the purpose of comparing not only the mature organs 

 of the hybrids with those of the parents, but also their origin and devel- 

 opment step by step so far as possible. As the study progressed it was 

 found that material for comparison of mature structures was abundant 

 enough, but material for the observation of the developmental stages, 

 which depends on the presence of mitotic figures, was not at all easy to 

 get, so that, as the second-generation hybrids were fairly rich in this 

 particular, the account of the development of the trichomes is practically 

 confined to plants of this, the second, generation. 



FIG. 19. Diagrams, based on numerous measurements, 

 showing reversion of the heads of short secreting tri- 

 chomes in the second-generation Juglans hybrids, 

 a, .7". californica X J. regia, plant A. Legend: solid 



line ( ), J. califcrnica: dotted line ( ), J. 



re(/ia; broken line ( ), hybrid. b, J. califor- 

 nica X J. nif/ra, plant 1, Legend: solid line ( ) 



J. californica; dotted line ( ), J. nigra: broken 



line ( ), hybrid. 



