10 HEREDITY AS ILLUSTRATED BY TRICHOMES. 



COMPARISON OF TRICHOMES OF THE OENOTHERAS. 



Upon comparing the extremes in length of the trichomes of the hybrid 

 with those of the parents we find (l) that the shortest pear-shaped trichomc 

 in Ocnothcra cruciata is 23.1 /"; the shortest in Oenothera lamarckiana, on the 

 other hand, is 37.8 /*, while the shortest in the cross is 29.4 M, and (2) that 

 the longest pear-shaped trie-home in Ocnothcra lamarckiana is 67. 2 /*, the 

 longest in the hybrid is 50.4 /*, and in Ocnothcra cniciata is 46.2 M. In every 

 instance the shortest pear-shaped trichomes occur on the lower surface of 

 the leaf and between the veins, although in Oenothera lamarckiana short 

 ones occur also on the other areas; the longest are to be found on the veins, 

 although in the hybrid long ones may occur on the stem as well. As will 

 appear repeatedly in this study, this relation of the length of trichome to 

 area where found is so consistent in all plants that there may be some com- 

 mon underlying' cause, as, for example, nutritive conditions, which induces 

 the differentiation. At any rate, the variation is constant and must be 

 taken into account in the comparison of the trichomes of related forms. 



Reference to tables 1,2, and 3 will show that in every instance where 

 analogous trichomes from similar areas are compared, those of the hy- 

 brid are intermediate in size, those of Ocnotlicra lamarckiana are larger, 

 and those of Ocnotlicra cruciata are smaller. In every instance also, the 

 trichomes of the hybrid are somewhat less than one-half the sum of the 

 other; that is, they are not average in measurement. In such hybrids as 

 the above, when both parents possess identical, or practically identical, tri- 

 chomes, whose sole apparent difference is that of size, we would perhaps 

 expect such a result. This would be based on one of at least two grounds: 

 First, either that the trichomes of the hybrid would represent the equal in- 

 fluence of both parents, as McFarlane* showed long ago for several hybrids; 

 or second, if there was inconsistent working' of the law of dominance so 

 that a portion of the trichomes would show reversion to one and a portion 

 to the other parent and the third portion be indeterminate in this regard, 

 the average of the whole would be intermediate. So far as the present 

 studies would indicate, there is no clear proof that reversion may not occur 

 as stipulated in the second alternative, since 60 per cent of the pear-shaped 

 trichomes of Ocnothcra lamarckiana come within the range of size exhibited 

 by these types in the hybrid, and 80 per cent of the pear-shaped trichomes 

 of Ocnothcra cniciata are within the range of size of those of the hybrid. 

 But it seems more probable, from the general intermediate condition of 

 the hybrid, that the trichomes may each also show the influence of each 

 parent in approximately equal measure. 



*A comparison of the minute structure of plant hybrids with that of their parents, 

 and its bearing on biological problems. Trans. Roy. Soc. Edinb., 37 : 203, 1892. 



