492 



ECOLOGY 



tage derived from root hairs seems to be the increase of permeable sur- 

 face, which sometimes is as much as five or ten times that of a hairless 

 root of equal size. The youngest hairs emerge a short distance behind the 

 tip; farther back they are of mature size, and still farther back they are 

 withered and dead. Most root hairs are ephemeral structures, lasting 

 only a few days or weeks. Indeed, the entire epidermis is soon sloughed 



off, and the hypodermis (here called 

 the exodermis) becomes the outer 

 layer of the root, which through cut- 

 inization is thenceforth relatively im- 

 permeable to water. The continual 

 dying of the older hairs, as new hairs 

 develop toward the tip, gives rise to 

 the migration of root hair zon^s, mak- 

 ing absorption possible from new soil 

 regions (figs. 703, 704). Furthermore, 

 the ever increasing development of the 

 root system is accompanied by a con- 

 tinual increase in hair development, 

 thereby enlarging the aggregate area 

 of absorption and the total absorption 

 capacity. 



When unimpeded, root hairs grow 

 at right angles to the root, showing no 

 reaction to gravity stimuli. The hairs 

 become variously gnarled and con- 

 torted (fig. 705) through contact with 

 soil particles, with which an intimate 

 cementation is effected by the trans- 

 formation of the outer layer into muci- 

 lage. So close is this attachment that 

 when a plant is pulled from the ground, considerable earth adheres to 

 the root hairs (fig. 700), which are more apt to break than to separate 

 from the soil particles; this indicates the important part played by root 

 hairs in anchorage, especially in seedlings. The adhesion of hairs to 

 soil particles is of still greater advantage in absorption, since most of 

 the available water surrounds the particles as a film, in which are also 

 most of the salts utilized by plants as food materials. The carbon 

 dioxid excreted by the root hairs assists in dissolving the soil salts. 



Figs. 703, 704. — Seedlings of wheat 

 (Triticum sativum) : 703, a seedling 

 soon after germination; 704, a seedling 

 four weeks older; the root regions, which 

 in 703 ate hairclad (r) and agglutinated 

 to soil particles, have become hairless in 

 704 (r'), while the younger portions (r") 

 deeper in the soil have hairs; note the 

 hairless root tips; the wheat grain (g) 

 remains in the earth, illustrating hy- 

 pogaean germination (p. 936). — After 

 Sachs. 



