A Botanical and Economic Study. 8 1 



differs somewhat from that of the primary root in having the 

 rear walls of the cells crescentically thickened. The paren- 

 chyma is in regular rows outside, but near the epidermis it 

 becomes irregular and consists of loose cells. 



The secondary roots arise from the stem, and in their later 

 origin are aerial. They appear at first as nodules, which 

 grow larger until the epidermis is finally ruptured by the 

 emergence of the rounded tip of the root. The root is posi- 

 tively geotropic, and grows downward into the soil. Before 

 entering the soil, however, gum is formed in large quantity 

 on the tip, and is of thick, treacle-like consistency. In the 

 aerial roots of palms, similar gummy matter is formed by a 

 breaking down and enormous swelling of the external cell 

 walls. Cell walls which have undergone this mucilaginous 

 modification take, when placed in water, the consistency of 

 gelatin, and when warmed appear to dissolve, forming thick 

 mucilage. It is apparent by analogy that the secondary 

 roots in Zea form a mucilaginous covering by a change in 

 their external cell walls. The formation of these roots is 

 as follows : 



Fig. 2, Plate XVI, shows the developing secondary root 

 before the point breaks through the epidermis and hypoder- 

 mis. Three superimposed hollow cones are found immedi- 

 ately beneath the two outer protective layers, the outer and 

 middle cones being separated by a cushion of parenchyma. 

 The outer cone is composed of actively growing cells with 

 the nucleus and nucleolus plainly visible. It corresponds evi- 

 dently to the calyptrogen layer of Janczewski. The inner 

 cones correspond to the periblem and plerome cylinders with 

 the outer layer of the periblem as the dermatogen or proto- 

 epidermis. The cells of the plerome, destined to form the 

 central vascular system, are much longer than broad, the 

 long axis anticlinal. 



Stem. — Comparison of an upper and a lower internode 

 of the stem shows the following differences : x The upper in- 

 ternodes have a larger number of small peripheral bundles 

 than the lower, and the epidermal layers have comparatively 



1 Strasburger, Botanische Prakticum, 109. 



