48 THE BIOLOGY OF FLOWERING PLANTS 



backwards from a point about a centimetre behind the 

 tip. When they die away the external cortical cells become 

 suberised or corky, and form an exodermis impermeable 

 to water ; the power of absorption is lost. As the result of 

 secondary changes the cortex may die and be sloughed off, 

 the protection of the living tissues within being taken on by 

 a secondary corky skin. 



A root hair is a protrusion of an epidermal cell, the 

 lumina of the two being continuous. It has a delicate wall 

 and moulds itself to the grains of soil which it touches, 

 establishing a very intimate contact. The external layers 

 of the wall are mucilaginous, and this favours close union 

 with the soil colloids ; indeed, the soil colloid may merge in 

 the colloid of the wall. If a seedling is uprooted, the grains 

 of soil come with it ; they cannot be washed off without 

 injury to the root hair ; the two are glued together. A 

 consequence of this is that whereas the root hairs which 

 have been produced on a seedling grown in a moist atmo- 

 sphere are perfectly regular, those produced in soil are 

 distorted by their contact with the soil particles. 



The root hairs grow out just behind the region of active 

 elongation, and the importance of this is obvious. Only 

 where growth has ceased is it possible for the hair to make 

 contact with the soil particles without injury. Certain 

 succulents (Crassulaceae, Cactaceae), however, bear root 

 hairs at the extreme root tip ; these are probably produced 

 after growth has ceased. 



Root hairs function only for a short time, dying off 

 behind along with the cell layer which gives rise to them, 

 at least in rapidly growing roots. It seems likely that at 

 periods of slow growth the hairs may be more persistent. 

 Recently Whitaker (1923) has shown that many herbaceous 

 Compositae have roots which bear root hairs over their 

 entire length : these hairs may persist and function for two 

 to three years. Roots with secondary thickening have hairs 

 only at the tip. When seedlings are transplanted the hairs 

 of the exposed roots die off, and are renewed as the plant 

 establishes itself. 



