ON THE OCCURRENCE OF ROOT-HAIRS ON OLD ROOTS OF HELIAN- 



THUS RIGIDUS. 



E. E. WATSON. 



The root-hair, as an organ of absorption, covers typically not more than 

 one or two centimeters of the root, a few millimeters above the root tip. Dur- 

 ing the progress of an investigation of the subterranean organs of certain 

 Heljanthi, I observed that the roots of several species of this genus produced 

 hairs on much older portions of the root. Conspicuous among these is H. 

 rigidus. 



The general habit of this" plant resembles that of H. occidentalis. The 

 parent plant sends out rhizomes in all directions from the base of the stem. 

 These attain a length of one to three decimeters. They do not grow out 

 horizontally, as do most rhizomes, but grow down at an angle of 45°, and then 

 curve upward until they reach very nearly to the surface of the soil, there 

 terminating in a rather large loosely constructed bud. The rhizome does not 

 bear roots except from a point about 5 cm. from the terminal bud. Here four 

 or five simple roots are produced, which grow straight down, and attain a 

 length of one or two decimeters. These roots are unusually rough and uneven 

 in appearance. 



Examination of transverse sections of these roots reveals the presence of 

 typical root-hairs thruout their entire length. The hairs present two peculiari- 

 ties ; first, they are unusually long, many being found that measured .5 mm. 

 or more ; second, a very considerable proportion of them are branched. No 

 septate hairs were found, and the branching is dichotomous. 



Leavitt, in the proceedings of the Boston Society of Natural History, Vol. 

 31, describes two methods of root-hair origin. It is to be noted that the hairs 

 from the older portions of the root are produced according to the second 

 method ; that is, they arise from a small wedge-shaped cell of the epidermis, 

 which has been formed by an unequal division of an epidermal cell, and the 

 formation of an oblique cell wall. 

 UniA-ersity of Michigan. 



21st Mich. Acad. Sci. Kept., 1919. 



