322 WALDRON— THE PEANUT 



tion change following a former evolved condition in root hair pro- 

 duction due to adaptation to changed environment? Corn and 

 many other plants produce none when put in water. (2) Other 

 plants produce root hairs near the tips of their roots by elongation 

 of the outer epidermal cell walls. Since many hair producing plants 

 cease to bear these when in contact with water or saturated soil, 

 it would seem to the writer that instead of being two stages of spe- 

 cialization, it is an example of two types of absorptive tissue depen- 

 dent on ecological factors. It is a more or less epidermal surface 

 extension for absorption, dependent upon the amount of causing 

 stimuli present. The hairless aquatic plants may have had hairs 

 at some time, and some do produce them when growing in dry soil 

 again. 



As to the cause for root hair production, PfefFer^^ states that 

 too little or too much water hinders, while darkness and contact 

 accelerate. Snow in an extensive investigation on the causes of 

 their development finds that they are accelerated by a retardation 

 of growth, by mechanical means, or substratum resistance, espe- 

 cially if the roots of such are allowed to grow in a moist atmosphere. 

 She finds that they are retarded by a saturated atmosphere at high 

 temperatures, by a lack of oxygen, and by a saturated soil; light and 

 darkness, however, have no material effect. 



Observations and Experiments with Root Hairs 



It was noted that if seeds were planted in a heavy soil and germ- 

 ination was retarded by lack of moisture, the hypocotyl would some- 

 times swell considerably and give off adventitious roots which 

 branched profusely. By drawing the soil away, after the radicle 

 had grown an inch or two, until the lower end of the hypocotyl was 

 well exposed, the upper part of the side, and of the adventitious 

 roots with their hairs could be kept growing in saturated air. This 

 gave a good opportunity to determine the effect of sunlight on the 

 growth of hairs, as compared with those on a few plants whose roots 

 were kept in the dark, but also exposed to the air. The foliage of 

 both sets of plants had the same leaf exposure, so that the activity 

 and growth were as near the same in all as possible. In all cases, 

 both in light and in darkness, the rosette hairs were found at the base 

 of the side rootlets and there was no marked difference in their size 

 or abundance. This corresponds to the results of Snow, except 

 that in her observations she noted a slightly longer growth in the 



