348 



THE QUARTERLY REVIEW OF BIOLOGY 



added to by the region of growth below 

 and in turn is giving rise to the older 

 portion of the root above, it is a region 

 which is migrating progressively farther 

 and farther into the soil . Continually the 

 root is adding new cells which give rise 

 to root hairs in the lower portion of this 

 region, while the older root hairs in the 

 upper portion are collapsing and being 

 sloughed off. The region thus presents a 

 series of root hairs, gradated according to 

 length, with the shortest near the distal 

 end and the longest at the proximal end. 

 The root hairs are thus usually short- 

 lived structures, being formed by con- 

 tinuous elongation for a few hours, and 

 then, after growth ceases, in most cases 

 collapsing and being sloughed off in a 

 few days. 



There are, however, some exceptions to 

 these rules. Cowles reports (7) that the 

 prickly pear cactus, Opuntia, has root hairs 

 to the extreme root tip. McDougal (52.) 

 has found them persisting over 15 or more 

 centimeters of the root length of certain 

 woody legumes, as the honey locust, red 

 bud, and Kentucky coffee tree, for a 

 period of several months. It is likely, 

 however, that these hairs persist upon the 

 root for some time after they die. He 

 reports that in all except the younger 

 stages they are thick-walled and brown in 

 color and about four times their typical 

 diameter. These root hairs, then, unlike 

 those of most plants, undergo some cell 

 differentiation, as well as elongation. 

 McDougal correlates the persistence of 

 root hairs in these plants with absence of 

 root nodules. Miss Whitaker (86), how- 

 ever, found persistent root hairs in other 

 groups than the legumes. She correlates 

 this habit with a suppression of growth of 

 the root in diameter. In a number of the 

 Compositae, as asters, golden rods, dah- 

 lias, chrysanthemums, etc., she found 

 them persisting even for two or three 

 years. 



The function of root hairs has been the 

 subject of discussion ever since they were 

 first figured and described in the latter 

 part of the seventeenth century by Mal- 

 pighi (50) and Grew (z8). These two 

 botanists, the one an Italian and the other 

 an Englishman, laid the foundations for 

 plant anatomy. They both found the root 

 hairs and studied them in a general way; 

 but they differed in their opinions re- 

 garding their function . Malphigi thought 

 that they were the organs of absorption of 

 the plant; while Grew considered that 

 the plant absorbs through its tip, es- 

 pecially through the root cap. From this 

 time on, for a century and a half, botanists 

 differed upon this point. Their evidence 

 regarding it was conflicting, owing prob- 

 ably to using different kinds of plants and 

 to inexact methods of experimentation. 



In 1837, however, Ohlert (5 7b) performed 

 experiments which seemed to be con- 

 clusive. He used peas, lupines, and mari- 

 golds, cutting off the hairless root tip and 

 covering the wound with lacquer. He 

 found that the plant grew normally, 

 showing that the root absorbs through the 

 lateral surfaces upon which the root hairs 

 were disposed. Meyen (56) in the next 

 year concluded that the root hairs were 

 the organs of absorption of the plant. 

 He pointed out that they increase the 

 surface of the root and extend the area of 

 absorption. This conclusion has been 

 accepted almost without question by the 

 leading botanists to the present day. 

 Persecke (62.) thought that they were 

 able to condense water from saturated air 

 and absorb it. Mer (54) in 1880, how- 

 ever, showed that they must be in con- 

 tact with liquid water in order to ab- 

 sorb it. 



In 1883, Schwarz (73), who made the 

 first extensive study of root hairs, con- 

 sidered their function more intimately. 

 He calculated that they increase the sur- 

 face of the root from 5.5 to 18.7 times 



