HYBRIDS OF JUGLANS. 37 



TRICHOMES OF JUGLANS CALIFORNICA X JUGLANS NIGRA, F t . 



Four types of trichomes occur on the leaves of this hybrid. These are 

 (1) the awn-shaped trichome, (2) the disk-shaped trichome, (3) the long- 

 secreting- trichome (of two sorts), and (4) the short secreting trichome. 

 The awn-shaped trichomes are unicellular and usually occur singly; they 

 are found upon the ventral surface only of the leaves and measure 211 /* 

 more or less in length. While this type of trichome is especially abundant 

 in young leaves, it persists to a degree, so that it may be found in mature 

 leaves as well. 



As in the pure species, the disk-shaped trichomes are composed of a 

 short stalk and a broadly expanded head, of which the center is depressed 

 (fig. 9). The supporting stalk is 2-celled; the head is of many, perhaps 

 always of 32 cells, but is only 1 cell in thickness. The origin and develop- 

 ment of the trichome were not shown satisfactorily in the material at hand, 

 so that a description of these processes in hybrids of the first generation 

 can not be given. Measurements, in /*, on the diameter of the trichome 

 and on the depth of the depression of the head were made as follows: 

 diameter, 107.1, 109.2, 105.0, 100.8, 92.4, 92.2; depth, 8.4, 8.4, 18.9, 

 16.8, 21.0, 12.6, respectively. 



The longer secreting trichomes measure 126 p more or less in length 

 and are of two types, which appear to be distinct. The more usual form 

 consists of a stalk of 4 cells and a head of 4 cells which are radiate. The 

 other form is composed of a stalk of a varying number of cells, usually 

 more than 4, and a flat expanded head of about 8 cells. Although both 

 forms occur on both surfaces of the same leaf, the latter is to be found 

 chiefly, perhaps, on the veins. The more common form will be described 

 first. After the young trichome has been cut off from the epidermis a 

 transverse wall appears which separates the head portion from the part 

 which will become the stalk. The next division of the trichome is also a 

 transverse one and occurs in the stalk-cell. The third division takes place 

 in the head-cell and is a longitudinal one. What the order of cell-divisions 

 was after this was not learned. The heads of the mature trichomes are 

 composed of rather long cells, so that in effect the head is neither like the 

 head of the long secreting trichome in Juglans nigra, nor like that in 

 Juglans calif ornica, but holds an intermediate position. The following 

 measurements, in /*, were made on the heads of mature trichomes: length, 

 21.0, 25.2, 25.2, 21.0, 25.2, 21.0, 33.6,23.1, 21.0, 25.2, 23.1, 25.2. The 

 average of these is 24.1 p. 



The early stages in the development of the second type of long secreting 

 trichome, that with a head of 8 cells, were for the most part not seen, but 

 certain peculiarities in the cell -divisions of the head may be recorded. In 

 transverse sections of the heads of all of the other trichomes studied the 



