LEAVITT: TRICHOMES OF ROOT. 
281 
guished from the other epidermal cells of the root.” A few notices 
of special hair-cells, observed in several plants, but without state¬ 
ments as to their origin, are to be found. The distribution of these 
structures in vascular plants seems not heretofore to have been con¬ 
sidered. 
In some plants the trichoblast not infrequently undergoes trans¬ 
verse division. Two or more trichomes may then stand in succes¬ 
sion in the same longitudinal cell series. In a few cases the 
trichoblast frequently or regularly divides longitudinally once (some 
species of Lycopodium, Luzula, Anigozanthos), or twice successively 
(Lycopodium, Lachnocaulon). In this way groups of 2, 3, or 4 
trichomes, standing side by side, are formed (pi. 16, fig. 22; pi. 17, 
fig. 55; pi. 18, figs. 75, 84) intercalated between the ends of neigh¬ 
boring non-piliferous cells. But in general, trichoblasts and the 
resulting trichomes stand singly amongst the other elements of the 
superficial layer. 
The atrichomic segment may or may not divide. For example, 
in Lycopodium the non-piliferous cells never divide so far as I have 
seen. In the maturer tissue, therefore, the trichomes occur regularly 
alternating with elongated hairless cells. In some groups, however, 
the latter at an early stage divide transversely and the transverse 
divisions are made in an orderly manner. In these cases the tri¬ 
chomes occur at regular intervals in the longitudinal rows, inter¬ 
spaced as a rule by a given number of elongated cells. The most 
pronounced instance observed is that of Azolla, in which the uni¬ 
formity of the divisions in different longitudinal rows often gives a 
zonal arrangement of the trichomes. The figures of Isoetes may 
also be referred to in this connection (pi. 17, fig. 23-26). 
Speaking generally of those groups and species in which tri¬ 
chomes of the second type occur, the formation of trichoblasts is 
not an inevitable event in the development of the epidermis. They 
may be more or less extensively intermitted for a time. They seem 
to be called out by some stimulus in the substratum. When the 
stimulus is inoperative the trichoblasts are not formed. In this 
event the roots are hairless. Thus Elodea is mentioned by De Bary 
as being destitute of root-hairs. Yet under proper conditions tri¬ 
chomes are produced. They are of trichoblastic origin. Nymphaea, 
also, is often given in the textbooks as a plant without root-hairs. 
But these organs may be found in abundance, borne by differentiated 
cells, on roots rapidly growing in soil rich in nutritive matter. 
