TRICHOMES IN PURE SPECIES AND HYBRIDS. 59 



cells than normal. All of these abnormal trichomes were found on the 

 veins of leaves, but were not very abundant. An additional form of ab- 

 normal trichome, which was plainly different from any trichome to be 

 found on the Juglans hybrid, and pure lines as well , was found in F, of both 

 hybrids. This type had a stalk 2 -ranked in place of 1 -ranked, as usually 

 is the case. The head also was 2 -celled and not 4 or more celled, as in 

 the other multicelkilar trichomes. This type had its origin in a trichome 

 rudiment which is divided at first long-itudinally, in place of transversely, 

 as is the case in the other trichomes. From this circumstance alone we 

 here observe the beginning of a new type of trichome, not by the modifi- 

 cation of a preceding' form, as is the case of the 6-celled and the 8 -celled 

 trichomes above mentioned, and probably the other multicellular trichomes 

 also, but by a sudden change in the sequence of cell -division. 



Numeroiis measurements, which were made on the length of the tri- 

 chomes and the length and the diameter of the heads, showed that there 

 is considerable variation in size, and further study revealed a definite 

 relation between this variation and the position occupied by the trichome, 

 or the age of the member bearing it. By keeping these facts in mind, 

 just comparisons could be made between trichomes placed in analog'ous 

 locations and under similar conditions, which otherwise would not be pos- 

 sible. The general facts of the variation as induced by such environ- 

 mental conditions may be expressed briefly thus: the trichomes and the 

 heads of trichomes which are situated on the veins of the leaves or in 

 their immediate vicinity are usually larg-er than trichomes of the same 

 sort which occupy a position between or relatively remote from the veins 

 of the same leaf; trichomes on young members are usually larger than 

 those on members that are old; trichomes from the dorsal surface are 

 usually larg'er than the corresponding trichomes from the ventral surface 

 of the same leaf. 



As regards reversion to ancestral characteristics, the trichomes of such 

 of the first-generation hybrids as were studied were intermediate in size and 

 hence did not exhibit reversion, and those of the second generation were 

 not uniform in this particular. It is probable that study of a large number 

 of plants of the second generation will show alternative inheritance. In the 

 second-generation hybrid, in which nigra enters, the disk-shaped and the 

 long secreting trichomes both show nigra characters and may be regarded 

 as reverting to that pure species, while as to these trichomes in the hybrid 

 in which regia enters the inheritance is not so clear. In this case, the disk- 

 shaped trichomes of the hybrid are intermediate in size between those 

 of californica and regia, while the heads of the short secreting trichomes 

 have the form of those in regia, but are much larger. Should the short 

 secreting trichome of the Juglans californica X Juglans regia be considered 

 a reversion, as probably is the case, we here have another indication that 



