PURE SPECIES AND HYBRIDS OF JUG LANS. 



25 



in the young than in the old leaf, which condition must be taken into con- 

 sideration in such a comparative study as the present one. 



JUGLANS NIGRA. 



The following kinds of trichomes are to be found on the leaves of Juglans 

 nigra: (l) awn-shaped, (2) disk-shaped, (3) short secreting trichomes, 

 and (4) two types of the long secreting trichomes (figs. 6 and 7). 



The awn-shaped trichomes measure 252 /* more or less in length and are 

 unicellular. They occur either singly or in groups of 2^ or more. The awn- 

 shaped trichome is especially abundant on the young leaves, where it is 

 found on the ventral surface only, although it persists to a degree, so that 

 scattering trichomes are met on the older leaves as well. 



The disk- shaped trichomes are relatively short and have broadly ex- 

 panded heads which are closely appressed to the surface. In longitudinal 

 section a disk-shaped trichome is seen to consist of a saucer-shaped head 

 and a stalk of 2 cells, although an occasional trichome is found which has 

 a stalk of 3 cells (fig. 6, ^). Certain stages in the development of the 

 disk-shaped trichomes were observed. It was seen to arise from the epi- 

 dermis as a squat projection which is soon separated from it by a transverse 

 wall. A second transverse division follows, by which the head is difiEer- 

 entiated from the stalk. The terminal cell next undergoes division by 

 longitudinal walls, so that a 4-celled condition results. The stalk-cell, by 

 the formation of a transverse wall, attains the adult number of cells, 2, 

 when cell-division of the stalk ceases. 



In two cases the failure to form a second transverse wall in the stalk 

 was noted. The cells of the head divide by the formation of longitudinally 

 placed walls only and are finally composed of 24 or more cells. The head, 

 therefore, is an expanded plate of cells 1 cell in thickness. The heads were 

 75 /A more or less in diameter and the depression of the head was 25 /ia more 

 or less beneath the rim. 



Table 8 gives in detail the diameter of the heads and the depth of the 

 depression in several representative and mature disk-shaped trichomes. 



