52 



HEREDITY AS ILLUSTRATED BY TRICHOMES. 



MEASUREMENTS ON THE TRICHOMES. 



The measurements which were made on the multicellular trichomes of 

 the pure lines and first generation oijuglans hybrids indicate, as above 

 shown, considerable variation in size, but comparative study showed also 

 that the causal relations attending - the variations might be traced, or at 

 least surmised. Thus the variations were seen to be associated with the 

 age of leaf or with the position occupied by the trichomes on the leaf. 

 These conditions were shown to hold good for all the plants brought under 



observation. It followed, 

 therefore, when these facts 

 were taken into account, that 

 comparison between closely 

 related forms was possible. 

 As the studies reported on 

 up to page 42 were made in 

 two successive seasons, 1906 

 and 1907, there remained the 

 possibility that variations in- 

 duced by seasonal differences 

 might in some degree alter 

 the measurements as given 

 in the earlier studies by modi- 

 fying the development of the 

 hybrids whose ancestors va- 

 ried in climatic experience as 

 well as in blood. There also 

 remained an instance of the 

 behavior of trichomes in re- 

 version to the pure lines to 

 be observed, since no hybrid 

 examined during the earlier 

 course of this study exhibited 

 this phenomenon. The meas- 

 urements to be given presently, therefore, are calculated especially to 

 show two things, namely, (l) whether the trichomes exhibit seasonal 

 variation in size, and (2) whether ancestral characteristics relating to 

 size of trichomes reappear in the second hybrid generation. 



JUGLANS CALIFORNICA X JUGLANS NIGRA, SECOND GENERATION. 



Of the several forms of multicellular trichomes, measurements were made 

 on the disk-shaped and on short secreting types only, and of these the 

 latter offered most favorable material and was mostly used. 



The following measurements, in /*, were derived from disk-shaped tri- 

 chomes taken from the largest and from the smallest of the hybrids. They 



Pig. 18. Abnormal and aberrant forms of trichomes of 

 Tuglans californica \ Juglans nigra, F 2, plant 1: a, 

 young, and b and c mature stages of a new type of 

 trichome (the first cell-division of the trichome is lon- 

 gitudinal in place of being transverse, as is the usual 

 position); d and e, abnormal trichomes. (a, b, and c, 

 X1200; d, X 840; e, X 535.) 



