PURE SPECIES AND HYBRIDS OF JUGLANS. 21 



become the head; all succeeding- divisions, therefore, differentiate the two 

 regions, namely, stalk and head. The second division is a longitudinal 

 one in the terminal, the head-cell, which is followed by a transverse divi- 

 sion of the stalk-cell. The stalk does not divide further in normal cells. A 

 single monstrosity was observed however (fig. 4, 0), in which the stalk 

 had 4 cells. The subsequent cell-divisions are confined to the region of 

 the head of the trichome . The second and third divisions of the head are 

 by walls placed at right angles to the preceding division- wall, so that the 

 head becomes 4 similar cells (fig. 4,/). 



Each quadrant is next bisected by a radial wall (fig. 4, ). After this 

 the sequence of cell-division appears not to be regular, although the 

 octants usually undergo radial division; and this process is repeated until 

 a head of about 32 cells is the result. Whether the mature trichome may 

 have more than this number of cells was not determined, but in some in- 

 stances the presence of cell-walls placed at right angles to the radial walls, 

 and parallel to the long axis of the trichome, would indicate a larger num- 

 ber. The head of the trichome never exceeds 1 cell in thickness. At an 

 early stage in the development of the trichome the peripheral portions of 

 the head-cells become somewhat enlarged, so that in longitudinal section 

 the edge of the disk is bluntly angular. This is the beginning of the rim 

 which is a characteristic of the disk-shaped trichomes. By the more active 

 growth of the lower portion of the more peripheral cells the rim is pushed 

 upwards, i. e., in a direction away from the surface of the leaf, until in 

 section the head of the trichome is markedly concave (fig. 4, ft) . 



There is much variation in the depth of the concavity of the disk, as in 

 some trichomes the tip of the trichome was quite flat, while in others the 

 concavity was pronounced. The diameter of the heads in the mature tri- 

 chomes was more uniform, as the following measurements, in /*, which 

 were made on representative trichomes selected at random, would indicate: 

 117, 100, 100, 117, 100, 109. 



The long secreting trichomes are 126 /*, more or less, in length (fig. 5). 

 The trichome takes its origin as a slender papillate projection of an epi- 

 dermal cell; it is early cut off from the epidermis by a transverse wall. 

 Subsequently it undergoes division, so that the young trichome consists of 

 3 cells, of which the terminal ones are the rudiment of the head of the tri- 

 chome. The third wall is, as in the short secreting trichome, almost surely 

 a transverse one in the stalk-cell. After this the sequence of cell-division 

 was not followed, but the final result is that the head is composed of 4 

 equal cells, radially disposed, and the stalk is composed of 4 lineally- 

 arranged cells. No further cell-divisions occur in this type of trichome. 

 The terminal cells which constitute the head become densely filled with 

 coarsely granular material; the cells of the stalk are poor in protoplasm. 



