3 6z 



THE QUARTERLY REVIEW OF BIOLOGY 



that Rheum, Brasska, Lepidium, and Plan- 

 tago are like Chenopodium in being posi- 

 tively chemotropic to phosphates. Oats, 

 wheat, barley, and millet react, however, 

 to ammonium compounds instead of 

 phosphates. Agrostemma, another grass, 

 reacts, on the other hand, only to nitrates. 

 Lychnis cornonaria and Oenothera, the evening 

 primrose, react both to nitrates and phos- 

 phates. Ornithopus reacts to phosphates 

 and to all potassium compounds. No 

 instance was found of a negative chemo- 

 tropism, that is, a curvature away from a 

 substance. All of the plants named above 

 react also to soil particles; but root hairs 

 of Urtica (a nettle), Nicotiana (tobacco), 

 and Verbascum (mullein) react neither to 

 soil particles nor to salts. He concluded 

 that the region of sensitivity and the region 

 of activity were both located in the tip of 

 the root hairs. He found no correlation 

 between the specific reaction of the plant 

 to these substances and its momentary 

 need of them. There thus seems to be no 

 possibility of a teleological explanation of 

 these reactions. It is apparent, however, 

 that they tend to lead the root hair in its 

 growth in the soil in the direction of in- 

 creasing concentrations of nutrients, and to 

 become appressed to soil particles, along 

 which these solutions migrate, and from 

 which their content is replenished by 

 action of root hair secretions and by 

 bacterial processes. 



THE RATE OF ELONGATION OF ROOT HAIRS 



It is rather surprising that with all the 

 study of root hairs in almost every other 

 possible aspect no attention has been paid 

 until now to the rate of elongation. The 

 only reference in the literature, prior to 

 the work of the writer is that of Reinhardt 

 in 1891 (65), who determined the increase 

 in length of root hairs of Lepidium during 

 a period of Z4 hours, and records his re- 

 sults by a simple statement that they grow 



from 0.3 to 0.7 micron per minute, and] 

 may attain a rate of 0.9 micron. Soko-I 

 Iowa (fig. 3) drew the form of the tip at I 

 1 minute intervals for iz minutes and thus 

 showed that the tip does not grow sym- 

 metrically but that the area of enlarge- 

 ment shifts from side to side slightly. 

 The writer has found, by measurement, 

 that while the rate is fairly constant over 

 hourly periods, it varies considerably for 

 ten minute intervals. The writer first] 

 measured (11) the rate of elongation of 

 root hairs produced on cuttings of Tra- 

 descantia fluminensis growing in Knop's 



Fig. 3. 00 Sketches made of the tip of a toot hair 

 of Brasska napus at successive one-minute intervals, 

 showing the form changes and shifting of the area of 

 cell wall extension during growth. (After Soko- 

 lowa.) (b~) Sketch of the form of the root tip re- 

 sulting. (After Sokolowa.) 



solution and in tap water respectively. , 

 Readings were taken at ten minute, or : 

 in some cases at longer intervals, over a 1 

 period of several hours. It was found ! 

 that the rate of elongation was more rapid I 

 in tap water than in Knop's solution. 

 Because of the variability of tap water, 

 and the complexity of the nutrient solu- 

 tion, these results were not of great sig- 

 nificance. 



Seedlings have, however, proved more 

 suitable for such studies, inasmuch as 

 they can be grown in small chambers on 

 the stage of the horizontal microscope. 





