62 HEREDITY AS ILLUSTRATED BY TRICHOMES. 



than in the former case, which was somewhat unexpected, inasmuch as in 

 certain instances {/uglans californica X Juglans nigra, for example) the 

 leaves of the hybrid far exceed those of either parent in the range of their 

 variability. The extreme variability of the leaves of the hybrid referred to 

 was estimated at 500 per cent, which has reference to size merely; if other 

 characters were measured the range would be found quite as great, while 

 the extremes in size of the trichomes of these leaves fall under 100 per 

 cent and usually much below that figure. 



A comparison of the trichomes shows in general that there is a direct 

 connection between the size and the condition under which they are placed. 

 For example, trichomes which are located on or close to the veins of a 

 leaf are larger than those of the same kind between the veins of the same 

 leaf, and also trichomes of the dorsal surface are usually larger than those 

 on the ventral surface. The difference in the size of the entire trichome 

 extends also to the portion active in secretion— the terminal cells of the 

 multicellular types. Owing to such relation between size and position oc- 

 cupied by the trichomes, it is probable that the variation is to be associated 

 with differences in physiological conditions, as nutritive relations, quite 

 as in the fluctuating variability of larger plant organs. 



In general it was determined that each species has trichonjes which in 

 form and in size are characteristic of the species, and should it chance, as is 

 usually the case, that both parental lines of a hybrid have trichomes type 

 for type the same, but differing only in size and form, the corresponding 

 trichomes of the hybrid, at least of the first generation and frequently in 

 later generations, hold some degree of intermediacy. But if one pure 

 line bears trichomes unlike those of the other pure line, the odd trichomes 

 are transmitted unchanged either in form or in size. The Solarium hybrid 

 was the only clear case of unilateral inheritance, although Juglans cali- 

 fornica X Juglans nigra almost surely has this type of inheritance also. 



Not only do the trichomes vary in size, but certain observations indi- 

 cate that they have an unequal distribution, so that if in size and form of 

 trichomes the inheritance may be said to be bilateral it may no longer 

 be considered such when the facts of distribution of the trichomes on the 

 members bearing them is taken into consideration. This condition was 

 especially noted in Oenothera hybrid. In the hybrid and in the pure parents 

 the trichomal distribution was observed to be as follows: The pear-shaped 

 trichomes in Oetiothera lamarckiana was found on leaf, on stem, and spar- 

 ingly, on capsule; in Oenothera cruciata the same trichome type was on 

 the leaf, stem, and abundantly on capsule; in the hybrid it occurs on leaf 

 and stem, but not on the capsule. The club-shaped trichomes in Oenothera 

 lamarckiana occur on leaf, stem, and capsule, but in Oenothera cruciata and 

 the hybrid this type is to be found only on the capsule. This observa- 

 tion, and others also, make it probable that the trichomal system is not 



