ROOT HAIRS AND GROWTH 



373 



rated air to a solution, and less frequently 

 in root hairs newly formed in the 

 solution. 



Wortmann (88) attributed these changes 

 to alterations in turgor pressure. Soko- 

 lowa (76) considered that they are due to 

 alterations in the oxygen pressure of the 

 medium. It is not unlikely that more 

 than one condition in the environment 

 may bring them about. 



The writer (fig. n) has attempted to 

 classify these changes in form under two 

 types, namely changes in extent of the 

 growing area at the tip of the hair, and 

 changes in the direction of growth. The 

 simplest type of modification is the so- 

 called swollen (fig. 11 (1)), in which there 

 is a temporary change in diameter and a 

 later resumption of the same diameter. 

 This type appears to be the most common 

 in solutions of low toxicity, and is the 

 one found by Jeffs for temperature changes. 

 The next type is the kinked, in which there 

 is a temporary change in direction with 

 a subsequent resumption of growth in the 

 original direction (fig. 11 (z)). These 

 two types may be taken as showing a 

 minimum of injury due to the changed 

 environment. The next is branching. 

 This consists in an increase in the extent 

 of the growing area followed by a restric- 

 tion again, as in the swollen, but now 

 with two loci instead of one. This 

 necessarily involves a permanent change 

 in direction of at least one of the branches 

 and usually of both and is not to be con- 

 fused with the duplex hairs discussed 

 above. Van Tieghem (82.) noted such 

 double hairs in three species of Distichia. 

 Haberlandt (Z9) reported it about the 

 same time, 1887, in Brassica napus and 

 other cruciferae. Hill reported it in hairs 

 growing in concentrated solutions (34), 

 and Miss Roberts (66~) figured it. The 

 writer (19) has noted it in calcium chloride, 

 but more commonly in calcium nitrate, 



where it is found in amphibious hairs 

 and also in the aquatic hairs developing 

 after immersion in the solution. 



The flask-shaped hair (fig. 11 (4)), 

 is not so common, but is an example of 

 response to the shock of immersion by 

 ■ enlarging the areas of growth, and then a 

 gradual recovery to normal diameter and 

 direction. The inflated hair is by far the 

 most common in very toxic solutions, and 

 in the most toxic solutions which will 

 support growth at all it is the only type 

 found. It consists in (fig. 11 (5)) in- 

 crease in diameter and continued growth 

 at this new diameter. Hence there is no 

 recovery to the original type. The most 

 extreme type of abnormality, as judged 

 on this basis, is the spatulate. It consists 

 of growth at a progressively greater and 

 greater diameter (fig. 11 (6)). 



Next to the types of abnormalities 

 which involve a modification followed by 

 a return permanently to the normal, there 

 are types of modification which display 

 a periodicity between the new and the old. 

 Such is for instance the beaded, in which 

 there is an enlargement followed by (fig. 

 11 (7)) a return to the original diameter, 

 then an enlargement, then a return, etc. 

 These were not found in calcium chloride, 

 but were not uncommon in calcium nitrate 

 of a high pH, namely 1 1 .0. A correspond- 

 ing type, but consisting of an alteration 

 of direction, instead of diameter, is the 

 undulating (fig. 11 (8)), which was very 

 much more frequent. 



Perhaps the most extreme type of all is 

 the so-called crooked, (fig. 11 (9)) in 

 which there is a progressive enlargement 

 or diminution of diameter, and change of 

 direction, without any apparent rule. 



The curved (fig 11 (10)) is a type in 

 which there is a progressive change of 

 direction. 



This classification of the abnormalities 

 of root hair form indicates that root hairs 



