COMPARISON OF TRICHOMES IN PURE SPECIES AND HYBRIDS. 63 



to be taken as a single unit, but rather that it is to be conceived as being- 

 comprised of as many units as there are distinct types. 



In most of the plants studied it was not practicable to study the origin 

 and development of the trichomes, either from a lack of suitable material 

 or because the trichomes were unicellular. But in Jug lam the material 

 for study was the most favorable, so that the ontogeny of the trichomes 

 was followed to a certain extent in all plants, both hybrid and pure species, 

 but particularly was this done in the second -generation material. In 

 Juglans both unicellular and multicellular trichomes are found, of which 

 4 types occur in californica and regia, and an additional type in nigra. 

 The hybrid strain with nigra blood has 5 trichome types, while that with 

 regia blood has but 4. The trichomes common to all Juglans studied 

 are (l) awn-shaped unicellular, (2) short secreting, (3) long secreting, 

 and (4) disk-shaped types. The extra form, found in nigra and its deriv- 

 atives, is a long type with a number of cells in stalk and head. The short 

 secreting trichome has 6 cells, the long secreting type has 8 cells, while 

 the disk-shaped form has 32 or more cells. The 6-celled trichome has a 

 certain sequence in development which it follows with perfect consistency, 

 which also is true of the 8-celled trichome. The latter trichome up to 

 the 6-celled stage has the same sequence in its cell-divisions as that of the 

 smaller form, and to these adds 2 others, which also have a proper 

 sequence. Therefore it seems probable that the two trichomes are espe- 

 cially closely related, and either that one developed out of the other or 

 that one represents an arrested stage of development of the other. 



The first 3 cell-divisions of the odd type of trichome, that peculiar 

 to nigra and its descendants, agree in sequence with the corresponding 

 stages in the short and the long secreting trichomes, but in the later 

 development it is different from either. Only the first 2 cell-divisions 

 of the disk-shaped trichome agree with the course of development of the 

 three trichomes just mentioned. This type of trichome, consequently, 

 probably is not so closely related to either of the preceding types as these , 

 particularly the first two mentioned, are to each other. 



In addition to the more common form of trichomes in Juglans, abnormal 

 forms and one aberrant type were observed both in the pure lines and in 

 the hybrids. The abnormal trichomes were very evident modifications of 

 the prevailing types; that is, they were short secreting trichomes with an 

 extra cell in the stalk, long secreting trichomes with a stalk of more than 

 4 cells, or (in trichomes with nigra blood) the odd trichome with more 

 than 8 cells in the head, or, finally, disk-shaped trichomes with a stalk of 

 3 cells. The aberrant trichome was observed in the second-generation 

 material of both hybrid strains. It had a structure quite different from 

 that of the other multicellular trichomes and it originated in a manner 

 peculiar to itself. It therefore is taken to represent a new type of tri- 

 chome, and is with little doubt a mutation. 



