STRUCTURE AND BEHAVIOUR OF ROOT-HAIRS 221 



form the most diverse angles with the vertical ; it thus seems a priori 

 probable, that the aforesaid localised origin of the root-hairs has nothing 

 to do with the stimulus of gravity, but rather represents a case of 

 correlation between the mother-organ and its appendages. 



Among Phanerogams every root-huir is a mere diverticulum of one 

 of the absorbing cells. It is obvious that the continuity of the cell- 

 cavity favours the rapid translocation of absorbed materials. The walls 

 of root-hairs are covered internally by a thin peripheral layer of proto- 

 plasm, which is usually somewhat thicker towards the apex of the 

 hair ; as a rule the nucleus lies either within this apical protoplasm or 

 close behind it. 



To begin with, every root-hair consists of an unbranched cylindrical 

 tube with a rounded tip ; this primitive form is, of course, only retained 

 if the root-hair develops in water or in damp air. It is under these 

 conditions, also, that root-hairs generally attain the greatest length. In 

 the various species examined by Schwarz the maximum length was 

 found to vary between T5 and 8 mm. To quote a few specific 

 instances, the maximum length in spp. of Pvtamogeton is 5 mm. (in 

 water), in Elodea canadensis 4 mm. (in mud), in Brassica Napus 3 mm. 

 (in damp air), in Pisum sativum and Arena sativa 2*5 mm., in Vicia 

 Fcdm *8 mm., in Muscari ootryoides *5 mm., etc. When growing in 

 soil, root-hairs usually suffer a very considerable reduction in length ; 

 the changes of shape which these structures undergo in contact with 

 soil-particles are of even greater interest, because they indicate the 

 physiological significance of the root-hairs in the clearest possible 

 manner. 



Every root-hair has an inherent tendency to grow out at right 

 angles to the surface of the root ; on its way through the soil, however, 

 it is certain before long to encounter a solid particle, which will force it 

 to turn aside. It then grows on in contact with the particle, until it meets 

 with a crevice filled with air or water, and is thus enabled to resume 

 its original line of advance. In this way a root-hair may come into 

 contact with the same solid object at several different points ; at each 

 contact the hair describes a knee-shaped curve, and at the same time 

 opposes the greatest possible surface to the obstacle. It consequently 

 often expands into a disc-like structure, or produces lobed outgrowths 

 which grasp the soil-particle like the fingers of a hand : the surface of 

 the hair, in fact, faithfully retains the imprint of all the irregularities 

 in the outlines of the objects to which it adheres (Fig. 84). These 

 changes in the shape of the root-hair are accompanied by a retardation 

 of its general growth, so that the hair often reaches a mere fraction of 

 the length which it could attain if it were allowed to develop in damp 

 air. The significance of this diminution in length is quite obvious. 



