18 



HEREDITY AS ILLUSTRATED BY TRICHOMES. 



JUGLANS REGIA. 



The leaves oijuglans regia are composed of from 2 to 3 pairs of leaflets 

 and a terminal one. The leaflets usually grade in size from the basal ones 

 to the terminal one, which is largest. They are broadly ovate and taper 

 rather abruptly to the apex, as well as to the base. The base of the 

 terminal leaflet is somewhat attenuated. The margin of the leaflets is 

 entire; the upper surface of the leaflets is minutely roughened or nearly 

 smooth, with the veins projecting somewhat above it (plate 3). 



Table 6 presents in a condensed form the leading characters of the leaves 

 of the 3 species oijuglans. 



Table 6. 



Leaves with 5 to 7 leaflets 



Leaves with 19 to 21 leaflets... J. californica. .. 



Leaves with 13 to 21 leaflets J. n 



Leaflets serrate J. californica. 



Leaflets entire 



Leaflets narrow ovate J. californica. 



Leaflets ovate 



Leaflets broadly ovate 



Leaflets, apex abrupt 



Leaflets, apex gradual taper J. californica. 



Leaflets, apex attenuate 



Leaflets, base abrupt 



Leaflets, base cordate J. californica. 



Leaflets, base gradual taper 



J. nigra. 



J. nigra. 



J. nigra. 

 J. nigra. 



J. regia. 



J. regia. 



J. regia. 

 J. regia. 



J. regia. 



TRICHOMES OF THE PURE SPECIES. 



There are 4, or perhaps 5 kinds of trichomes in the pure species of 

 /uglans studied, all of which are found in each of them, which are trans- 

 mitted kind for kind to the hybrids, so that all types are to be found in the 

 cross in the first and, so far as observed, in the second and the third genera- 

 tions also.* The trichomes will be called (l) awn-shaped, which may form 

 groups which constitute stellate trichomes; (2) disk-shaped; (3) long 

 secreting trichomes, apparently of two kinds; and (4) short secreting 

 trichomes. All of the trichomes, except the awn-shaped, are glandular 

 (figs. 4, 5, 6, 7, and 8). 



The trichomes are distributed in time and in space in a manner which 

 is fairly consistent and characteristic. In the youngest leaves the most 

 abundant trichomes are the awn-shaped ones. In old leaves, although not 

 plentiful, the short secreting trichomes may be the only trichomes, or 

 nearly the only ones, to be found. In what may be called the leaves of 

 middle age, all of the trichomes occur, none perhaps predominating. All 

 the glandular trichomes are to be found on both surfaces of the leaves, but 



*In the present study observations have been confined to the trichomes of young 

 and old leaves; only in one or two cases has the study been extended to the stem. No 

 attempt has been made to treat the general distribution of trichomes either of the pure 

 lines or of the hybrids. 



