LEAVITT: TRICHOMES OF ROOT. 
279 
Nuculiferae : Coleus sp., Myosotis palustris, Scutellaria lateriflora. 
Contortae: Limnanthemum nymphaeoides , L. sp., Menyanthes 
trifoliata, Sabbatia sp. 
Campanulinae : Lobelia Dortmanna , L. cardinalis. 
Synandrae: Aster sp. 
Trichomes of the Second Type. 
Trichomes of the second type originate early in the development 
of the piliferous layer as specialized elements. Only cells which 
are predetermined for the purpose in the young epidermis bear 
root-hairs. Differentiation begins with an act of division taking 
place in a cell of the embryonic tissue near the growing point. 
The nucleus assumes a position away from the center, toward 
one end of the cell (pi. 17, fig. 31, T). After the completion of 
nuclear division the new transverse wall cuts off a segment which 
in many cases comprises no more than a fourth or even a fifth of the 
original cell. The segmenting wall is often curved, whether viewed 
in longisection or tangentially (pi. 16, figs. 20, 21). It is often 
oblique, so that the segment in question appears more or less wedge- 
shaped. The new cell is at once seen to differ from its larger sister 
cell in its optical and staining qualities. In the living state the 
shorter cell appears more highly refractive. Its cytoplasm and 
nucleus stain more deeply in prepared sections. 
By the division of the original epidermal cell, therefore, there is 
effected a differentiation which is manifested immediately in the size, 
shape, and contents of the two resulting cells. The subsequent 
development of these two elements fulfils the expectation which may 
be formed from their initial differences, the anticipation that they 
will play very dissimilar parts, structurally and functionally, in the 
mature epidermis. The larger cell of the pair elongates considerably, 
sometimes enormously. In many cases it habitually divides trans¬ 
versely, once or several times (pi. 16, fig. 9). It is non-piliferous. 
The shorter and denser cell, on the contrary, elongates but little as 
a rule. It very seldom divides transversely. Very soon, in the 
zone of increasing elongation of the young root trunk, it becomes 
papillate, and later piliferous (pi. 16, fig. 1-3). 
The shorter, often wedge-shaped, often lenticular, and generally 
denser segments resulting from differentiating divisions in the 
