1905] SNOW—DEVELOPMENT OF ROOT HAIRS 39 
p- 70), and MER (53, p. 1279) speaks of the roots becoming accus- 
tomed to the medium. . 
The curving of corn roots in water is, according to Miss BENNETT 
(6) not aerotropic. BEAUVERIE (4) considers the turning up of 
water roots to be due to negative hydrotropism, for by using physio- 
logically dry solutions he was able to get them to grow downward. 
In an experiment in which a slow stream of tap water was passed 
into the bottom of a vessel in which the roots of corn seedlings were 
growing, every one turned down, and grew straight and entirely 
smooth. The stimulus may have been a rheotropic one, or it may 
‘have been the presence of fresh aerated water which caused the 
omission of the hair zone. 
An apparent exception to the explanation offered appeared in 
one root of sunflower grown in 0.5 N saccharose solution, in which 
the epidermal cells were shorter than the inner ones and still pro- 
duced hair. Close to the tip, however, the papillae were found on 
cells shorter than the cortical cells, which makes it seem probable 
that the epidermal cells on the upper part of this root were shorter 
than the cortical cells from the start, as is the case with Elodea. 
In this plant the epidermal cells at the tip are very much shorter 
than those of the inner cortex, and the difference does not entirely 
disappear as the root grows older. Consequently there is not the 
same relation between the epidermal and cortical cells when hair is 
produced, as there is in corn. Measurements of the cells of roots 
of Elodea growing in soil, quartz, and water give the following 
averages in millimeters: 
Medium ~ Cortex Epidermis Difference 
Oe yee ea: 0.100 0.068 0,032 
Oidrizs, 6253. 0.110 0.077 0.033 
WOMET oe 0.160 0.104 0.056 
Upon examination of the table the greatest relative length of the 
inner cortical cells is seen to be in water, and the least in soil, with 
the hairs in inverse relation, as was the case with corn. 
On the concave side of curved roots of corn the epidermal cells 
are shorter than the inner ones and at times show more hairs (jig. 
