HYBRIDS OF JUG LANS. 



39 



first three cell-walls are so arranged that a + -shaped figure results. In 

 this type of the long- secreting trichome, however, in which the 4, 5, and 

 6 cell stages of the head were seen, the arrangement of the walls is quite 

 different and indicates that there may have been a very different sequence 

 in the laying down of the walls from what ordinarily occurs. As nuclear 

 division was not seen in the heads, however, what the sequence of cell- 

 division was can not at present be told. In length of heads this type of 

 long secreting trichome is not different from the type having a head of 4 

 cells. The following measurements, in p, were made: length, 21.0, 21.0, 

 25.2, 29.2. The diameter of such heads, however, is usually considerably 

 greater. 



The short secreting trichomes were found on both surfaces of all leaves 

 but the oldest where they were apparently absent from the ventral side. 

 In length they range from 35.7 /* to 54.6 /* and the size is evidently to be 

 associated either with position on the leaf or with its age. The heads also 

 of the trichomes in both pure species vary greatly in length, and here, 

 also, the variation is probably in some manner connected with the position 

 occupied on the leaf, or with the condition of the leaf, but when taken into 

 consideration does not vitiate a comparison of similar organs which occur 

 under analogous conditions. The material studied did not permit an obser- 

 vation of the development of this type of trichome, so that for the present 

 a description of it must be omitted. The mature trichome consists of a 

 2 -celled stalk and a head of 4 cells placed radially. The first division of 

 the young trichome probably separates the stalk region from the head 

 region, as in both the pure lines, and the divisions of the head are longi- 

 tudinal. Measurements on the short secreting trichomes are given in 

 table 16. 



TABLE IB. Measurements on the short secreting trichomes of Juglans calif ornica 



X Juglans nigra, F^. 



*The average length of these heads is 19.8 /u; the average in diameter is 25.1 

 tThis type of trichome is absent from the ventral surface of old leaves. 

 tVein. 



