50 GROUPS OP TISSUES, OR TISSUE SYSTEMS 



ceils 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 

 Fig. 21.— Root hair, glandular hair, and Upward through the plant 



branched hair, hair of nettle. „ , i i 



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. 



