CHAPTER I — ROOTS AND RHIZOIDS 



I. SOIL ROOTS AND ROOT HAIRS 



Root hairs. — General remarks. — Most land plants possess roots which 

 branch freely and penetrate the soil in all directions. While the main 

 roots of adult plants often are large and stout, the 

 ultimate branches are slender and give rise to very 

 delicate organs, the root hairs. The chief r61e of 

 soil roots is in connection with absorption and 

 anchorage. The entire root system is concerned in 

 the latter, but the admission of water and salts is 

 restricted practically to the very youngest portions. 

 The structure and role of root hairs. — Root hairs 

 are extensions of the epidermal cells of roots (figs. 

 700-702, 705), most such cells possessing the ca- 

 pacity of developing hairs, though many are with- 

 out them.^ The walls 

 are thin and of cellulose, 

 readily permitting the 

 entrance of water and 

 solutes. Though root 

 hairs vary greatly in 

 length and abundance, 

 they average rather 

 more than a millimeter 

 in 1 ngth, and as many 

 as three hundred may 

 develop within a square 

 millimeter. While the 

 entire root epidermis is 

 permeable to water and 

 solutes, the chief advan- 



' In various pteridophytes (as Azolla) and monocotyls, however, hairs arise only 

 from special small cells rich in protoplasm. 



491 



Figs. 700, 701. — 

 Seedlings of mustard 

 {Brassica alba) ; 700, a 

 seedling dislodged from 

 the soil, showing par- 

 ticles of earth adhering 

 to the root hairs, the root 

 tip being free from hairs 

 or attached particles; 

 701, a seedling grown 

 in moist air, showing a 

 primary root with its 

 zone of root hairs, the 

 younger hairs toward 

 the tip being progres- 

 sively shorter ; the seed- 

 leaves (cotyledons) rise 

 above the soil, illustrat- 

 ing epigaean germina- 

 tion (p. 936). — After 

 Sachs. 



Fig. 702. — A longitu- 

 dinal section through the outer 

 portion of a root of the Wind- 

 sor bean {Vicia Faba), show- 

 ing a root hair (r) arising as 

 an outgrowth of an ordinary 

 epidermal cell, the nucleus 

 («) migrating into the hair; 

 highly magnified. 



