PURE SPECIES AND HYBRIDS OF JUGLANS. 31 



The awn-shaped trichomes occur on the ventral surface of the leaves 

 only and are especially abundant on young leaves. They sometimes are 

 found in groups, especially in young leaves, but in older ones they usually 

 occur singly; they measure 126 /*, more or less, in length. 



As distingnished from the awn-shaped trichome, which is unicellular 

 and non-secreting, the other forms are glandular and are multicellular. 

 The mature disk-shaped trichome consists of a 2-celled stalk and a flat- 

 tened head, 1 cell in thickness, of about 32 cells. The youngest stages in 

 development were not seen. After the head has been differentiated and 

 divided into 4 similar cells, each quadrant becomes divided unequally by 

 radial walls, so that a head of 8 cells results (fig. 8, c). Later each octant 

 is divided by walls which extend outwards from the newly-formed octant 

 walls and a head of 16 cells is the result. The actual formation of walls 

 subsequently was not observed. Usually the cell-division in the head 

 proceeds in a regular manner, so that the product is symmetrical, but a 

 single monstrous trichome was seen in which this had not been the case. 

 The history of this trichome, naturally, could not be studied; it is shown 

 at /, fig. 8. 



Measurements on the disk-shaped trichomes show that they vary both 

 in diameter and in depth in a regular manner, which is to be related to the 

 relative age of the leaf where found. Following are given measurements, 

 in /*, on the diameter and the depth of disk-shaped trichomes on young 

 leaves: diameter, 105.0, 109.2, 105.0; depth, 8.4, 8.4, 12.6, 16.8, respec- 

 tively. The trichomes are not so abundant on old leaves and, as the fol- 

 lowing figures (/a) show, they are also of less diameter; the depth is not 

 given: diameter, 58.8, 50.4, 46.2, 46.2, 58.8, 46.6, 42.0, 42.0, 50.4, 58.8, 

 averaging 45.0. 



The long secreting trichome measures 96 /*, more or less, in length. 

 The mature trichome consists of a head of 4 cells and a stalk of 4 cells 

 (fig. 8, y to m). It takes its origin as a papillate projection of an epider- 

 mal cell which is early cut off from the epidermis by a wall parallel to it. 

 The young trichome next undergoes transverse division, by which the por- 

 tion which is to become the head is differentiated from the portion which is 

 to be the stalk. The second division and the third division were not seen, 

 but are probably longitudinal in the head rudiment and transverse in the 

 stalk rudiments, respectively, as in the long secreting trichomes of the pure 

 species, as well as hybrids, where the sequence has been carefully followed. 



Either following the divisions by which the head becomes 4-celled, or 

 pari passu with these, the two final divisions of the stalk take place. Of 

 these divisions that of the outer cell occurs first. When the trichome is 

 mature the head is relatively long, but the material at hand was not favor- 

 able for a comparative study of the long secreting trichome, so that a more 

 precise statement can not at present be made. It was almost entirely 

 absent from old leaves and not very abundant on the young ones. The 



