ROOTS AND RHIZOIUS 495 



tage therefrom would appear to be in the facilitation of the oxidation 

 and the removal of deleterious substances. It seems likely that sub- 

 stances given off by plants of a particular species often are more injurious 

 to plants of their own kind than to plants of other species, a fact that 

 may help explain the value of crop rotation. 



Variations in the form and the development of root hairs. 'Usually, 

 when a seedling is transplanted, the hairs die at once, thus reducing 

 the amount of water admitted. Wilting soon occurs, and it is only when 

 new hairs have grown that the plant revives. Root hairs developed in 

 moist air collapse when brought into a drier medium. 



When maize roots are grown in water, hairs commonly are absent, 

 while a vigorous growth, apparently exceeding that in the soil, is 

 obtained in moist-air cultures (fig. 706). The same is true, perhaps, of 

 the majority of plants, but there are many exceptions, as in wheat and 

 oat seedlings, though the latter usually fail to develop hairs in water 

 after the food within the seed is exhausted. Wheat and oat seedlings 

 also have hairless roots in concentrated solutions, probably because 

 the latter inhibit the absorption of the water necessary for hair develop- 

 ment. 1 Oxygen is necessary for the development of root hairs, and 

 it may be that their absence or weak development in ponds and swamps 

 is due in part to low oxygen content. 



The exact factors that determine root hairs are not certainly known, but they 

 appear to develop best when root growth is retarded, and especially when retarda- 

 tion is differential. When roots elongate rapidly, epidermal cells tend to elongate 

 in the same direction, but when roots elongate slowly, epidermal cells grow trans- 

 versely, developing into hairs. Root elongation near the tip probably is so rapid 

 that the epidermal cells grow chiefly in a longitudinal direction. A short distance 

 back, cortical elongation ceases, but the epidermal cells, continuing to grow, develop 

 transversely rather than longitudinally. Probably the occurrence of hairs at the 

 root tips of certain xerophytes is due to slow root growth. Differential growth 

 often leads to the twisting of the root in water cultures, and hairs may occur on 

 the kinked portions. 



Maize roots have hairs in water if elongation is mechanically retarded; this may 

 explain why some hairless water roots, as in Elodea, develop hairs as soon as they 

 enter the soil (maize roots, however, are hairless in saturated soil). Contact with 

 solid bodies also may favor hair development. High temperatures favor both 

 water entry and root growth, but are detrimental to the development of root hairs, 

 further strengthening the root retardation theory. Sometimes there seems to be 

 a correlation between root hairs and shoots. An irrigated seedling of the creo- 



1 However, some salt marsh plants, such as Salicornia and Zostera, have an abundance 

 of root hairs, showing that different species react differently to similar stimuli. 



