360 



THE QUARTERLY REVIEW OF BIOLOGY 



2.9. 5°C, and none at higher temperatures. 

 Corn likewise produces none above Z7.o°C. 

 Jeffs (36) studied the effect of varying the 

 temperature upon the rate of elongation 

 of a single hair. He found that a very 

 slight change, two or three degrees, in 

 either direction caused either a temporary 

 or a permanent cessation of growth. In 

 many cases where growth was resumed, 

 there was apparent an enlargement of the 

 root hair during the period of adjustment 

 to the new temperature. Schwarz (73), 

 PfefFer (63), Went (85), and Miss Snow 

 all agree that contact in itself is not a 

 factor in root hair production, but that the 

 effect is a chemical one. 



The most extensive study of root hair 

 production has been upon the effect of 

 water and air respectively. Martin found 

 them shortest in water, longer in soil and 

 longest in air. Mer (53), who was one 

 of the first to consider this question, at- 

 tributed the lack of root hairs on many 

 species in water to the stimulating effect 

 of the medium upon root elongation. He 

 found that roots in air have root hairs 

 developing much nearer the tip than those 

 in water; that is, the region of elongation 

 of the root is greater in water. He con- 

 cluded therefore that the production of 

 root hairs was a response to inhibition of 

 root elongation. He also pointed out an 

 inverse .relation between root-hair pro- 

 duction and production of lateral roots 

 which was later confirmed by Constantin 

 (4) and Lesage (48). Miss Snow found 

 that corn, which normally does not pro- 

 duce root hairs in an aqueous medium, 

 may be induced to do so by increasing the 

 oxygen supply of the water in which it is 

 grown. Vochting (83) found that a 

 reduction of the 2 content of the air to 

 3 per cent inhibited the production of root 

 hairs in water. Surprisingly few species 

 of plants have been reported as producing 

 root hairs in water. The writer has 



compiled (ix) a list from the literature, 

 and added several, making a total of only 

 87 species. It is not considered, however, 

 that this list is by any means complete. 

 Many plants, which have been reported 

 as not producing root hairs in water, wil] 

 undoubtedly be found to do so, when the 

 proper adjustment is made of oxygen con- 

 tent, hydrogen ion concentration, osmotic 

 pressure, and chemical composition of the 

 solution. Mer (54) found that even onion 

 roots will produce root hairs if left in ait 

 long enough. 



Not much evidence has been accumu- 

 lated as to the effect of highly concen- 

 trated solutions, that is solutions of high 

 osmotic pressure of the medium, upon 

 root hair production. Patschovsky (60) 

 found that rhizoids of Cbara are inhibited 

 by an 8 per cent Knop's solution, whereas 

 they grow well in a 1 per cent solution. 

 The writer (13-Z4) has studied the rate of 

 elongation of root hairs of collards in 

 different concentrations of calcium chlo- 

 ride and calcium nitrate. He found that 

 above the optimum concentration, about 

 o.ozo molar solution, the rate falls off 

 at first rapidly with increased concentra- 

 tion, and then more gradually, reaching a 

 maximum concentration in the case of 

 chloride at about 0.185 m olar solution. 

 If, instead of adding additional salt to 

 the optimum concentration, which is 

 about equivalent in osmotic pressure to a 

 1 per cent Knop's solution, sugar be 

 added, it is found that a similar drop in 

 rate of growth occurs to that which is 

 obtained by adding equimolar amounts of 

 the salt. It is therefore concluded that 

 above a certain concentration osmotic 

 pressure of the solution does have a 

 retardative effect upon root hair elon- 

 gation. 



Krassnow (42.) and Gerneck (2.7) both 

 found that roots were much more abun- 

 dantly haired in Ca(N0 3 ) 2 than in KNO3. 



