2 HEREDITY AS ILLUSTRATED BY TRICHOMES. 



able for such an investigation, for the reason, which is apparent, that they 

 are either entirely inclosed by other tissues or so intimately related to such 

 other tissues that the extent as well as the manner of development is 

 largely conditioned by the development and position of such contiguous 

 Itissues. From the nature of things, structures so enveloped can, to a very 

 imited degree only, whether representing "units" or not, develop as inde- 

 pendent organs. However, even in wholly immersed structures, as the 

 conductive system, the single-structure method of approach has been pro- 

 ductive of rich results. But for the purpose of the present study it was 

 essential that the tissues compared be as independent of other tissues as 

 possible, and, in addition, should possess well-marked characteristics, both 

 as mature structures and in embryonic stages; hence it was decided to 

 select for the comparative study, as being most likely to give satisfactory 

 results, the trichomal system. 



The immediate objects of the investigation were, in brief, to study the 

 origin, development, inheritance, and variation, and, so far as practicable, to 

 observe the causes of variation of the trichomes in several pure species and 

 the hybrids of the species. In addition, it was desired to trace the behavior 

 of the structures in as many plants as possible, to the end that something 

 exact might be known as to the relation between such tissues and unit- 

 characters; that is, to be more specific, it was proposed to learn, for ex- 

 ample, whether the trichomal system as a whole constitutes a single unit, 

 or whether each separate type of trichome of a plant is to be considered a 

 distinct and independent character. 



PURE SPECIES AND HYBRIDS EXAMINED. 



The following hybrids, together with the pure lines, were studied: 

 Juglans calif ornica X Juglans nigra, Fj, Fj, and F3; Juglans calif ornica X Jug- 

 lans regia, Fi, F2, and Fs; Oenothera lamarckiana X Oenothera cruciata, Fj 

 andFj; Papaver somniferum X Papaver orientale , F,; Papaver somniferum X 

 Papaver pilosum, Fj; Solanum villosum X Solanum guinense, Fi, Fj, and 

 later generations. 



The material for the study of the trichomes of the Oenothera hybrid was 

 collected at the New York Botanical Garden in August, 1906, where it had 

 been formed three years previously. In this cross* the characters of the 

 pollen parent {Oenothera cruciata) are said largely to dominate, although 

 none appear to be transmitted unchanged. Particularly in the flower there 

 isasingularblendingbf the characters of both parents, as, for example, some 

 of the hybrids have short stamens, lamarckiana-\ike, and are incapable of 



*Mutants and Hybrids of the Oenotheras, by D. T. Macdougal and others, Carnegie 

 Institution of Washington Publications 24 and 81, 1905. 



