50 GROUPS OF TISSUES, OR TISSUE SYSTEMS 
cells at a distance of a few millimeters from the tip grow 
out into long, normally unbranched, thin-walled hairs, 
whose lumen is continuous with that of the main body 
of the cell. These root hairs are not cutinized, or only 
so at the base. They may attain a length of two or three 
centimeters but are mostly not over one centimeter in 
length and often much less. The thin wall is lined by a 
delicate layer of cytoplasm and the central vacuole is 
very large. These hairs push in between the particles 
of soil and lie in the film of water with which these are 
covered, absorbing some of this water by osmotic action. 
Such mineral salts as are in 
solution in this soil water in 
greater concentration than 
that of the same salts in the 
cell sap diffuse into the cell 
Fie. 21.—Root hair, aga peor dink and upward through the plant 
branched hair, hair of nettle. 
except so far as the plasma 
membrane is impermeable to them. 
71. The hairs on those parts of the plant exposed to the 
air may be continuous with the epidermal cells from 
which they have arisen, but mostly are separated from 
them by cross partitions. They may remain one-celled 
or may become many celled by cross septa. Sometimes 
they are much branched or merely bifid or stellately 
divided. In some cases the end cell of a short hair 
divides by vertical partitions in several planes to form a 
shield-shaped structure. Some hairs have the terminal 
cell enlarged and functioning as a gland which secretes 
sticky or oily substances. Certain hairs (as those of 
nettles) contain strong irritant poisons. The tip of the 
hair penetrates the skin of animals coming in contact with 
the plant and then breaks, permitting the poison to be 
forced out into the skin. 
