MORPHOLOGY, GENETICS AND BREEDING 33 



the foliage leaves laid down in the seed, but no new growth is visible 

 until about 21 days after planting. 



ROOT 



In a young peanut seedling the root is sharply differentiated from the 

 hypocotyl by an abrupt constriction (figures 2 and 3). This constric- 

 tion or collar marks precisely the transition zone (59, 76). Just at this 

 collar the intact epidermis of the hypocotyl gives way to the non-epi- 

 dermal outer layers of the primary root. Even superficial examination 

 reveals the fact that no true epidermis exists on the peanut root. Figure 

 2 shows the soil clinging to the broken, sloughing surface. This condition 

 in peanut roots, and its expected morphological corollary, the absence of 

 root hairs, have been observed by Pettit (50) and Richter (59). Richter 

 suggested that the ragged, uneven surface of peanut roots, where the 

 root hairs would normally be expected to occur, served as an active ab- 

 sorptive surface. Waldron (75) reported root hairs on peanilts and 

 showed that under ordinary conditions they usuallyt appeared as rosettes 

 of hairs at the junctions of the lateral and main roots; but Reed (55) re- 

 ported that few hairs occurred under field conditions. The internal de- 

 velopment and differentiation of tissues in the peanut root were first 

 touched upon by Richter in 1899. Yarbrough (76) has recently published 

 a description. of their internal development. 



A longitudinal section through the growing tip of a young root 

 shows the root cap, root initials, stele, and cortex. Among the root initials 

 there is no specific differentiation into initiating layers. The root cap, 

 cortex, and stele are laid down by the initials most proximal to these 

 areas by appropriate longitudinal, tangential, or radial divisions. 



The absence of epidermal initials ultimately results in the absence of 

 an intact epidermis and, consequently, the almost complete lack of root 

 hairs. The cross section in figure 4 shows that outside the cortex there 

 is a cork cambium-like region which by both radial and tangential cell 

 divisions lays down the closely packed absorbing cells which Richter be- 

 lieved substituted for root hairs as absorbing surfaces. The sloughing ex- 

 ternal cells are thus continually supplied from within. This region of the 

 root extends only 8 to 10 mm. back from the root cap and has been shown 

 to be an actively absorbing region in dye absorption studies by both 

 Richter and Yarbrough. Yarbrough emphasizes the probability that the 

 importance of this region. in absorption is confined to mineral ions, a fact 

 of considerable significance when one considers the ultimate position of 

 such maristems in the mature root system. With respect to the limited 



