Structural Botany ; Root Mechanism. X. 



Secondary Thickening is adjusted to the primary construction ; no residual 

 meristem (procambium) being left in the stele as first outlined. Division by tangential 

 walls is first observed in the conjunctive parenchyma immediately internal to the primary 

 phloems ; and this continues to give each a definite cambium tract, from which normal 

 differentiation ensues, as xylem products on the inside (centrifugal), and phloem units 

 on the outside: such extension naturally pushes the protophloems farther out, and 

 these are soon crushed out of existence. Similar tangential divisions follow in the 

 pericycle units external to the protoxylems ; and the preceding partial cambium arcs 

 are linked up in a continuous system, more or less wavy at first, but soon adjusting to 

 a definitely circular cambial zone, similar to that of the stem, but leaving the proto- 

 xylems on the inside more or less unaffected. Quite old roots may be thus distinguished 

 from stems in section by tracing the original protoxylems. The new tissue opposite 

 the protoxylems commonly remains parenchymatous as 'primary ray', rendering the 

 pattern more conspicuous. With a normal cambium in working order, growth follows 

 on as in the stem, and annual rings are produced in successive years ; but the first ring 

 is complete in the first year. The cortical region may fail to keep up with the new 

 growth, and is (in a tree-type) commonly exfoliated, including the endodermis. 



Phellogen giving a cork-formation, as in stems, normally follows from the outer 

 layer of the pericycle, which thus in old roots looks very much like a cortex. Secondary 

 tissues with vessels, secondary medullary rays, sieve-tubes, fibres, &c., agree with the 

 details of the corresponding stem for different types. 



Lateral Hoots : Ramification of the stem follows the lines of pre-existing 

 organization, and the laterals are thus associated with the very definite plan of leaf- 

 arrangement as repeating an ancestral mechanism of construction : ramification of the 

 root, in absence of leaves, follows pre-existing organization as that of the protoxylem 

 centres. Owing again to the special mechanism of the absorbing peripheral layers, 

 laterals are endogenous ; they are initiated in the pericycle, opposite the protoxylems, 

 and are hence put in immediate communication with the water-conducting tissues. 



Tangential divisions are initiated in a few cells of the pericycle, thus delimited 

 in transv. sect., but irregularly spaced in the longitudinal dimension, as a rhizogenic 

 plate. Two sets of such divisions suggest the 3 tiers of apical meristem (as calyptrogen, 

 periblem, and plerome), and the new apex is fully constituted; the young root extending 

 radially to reach the surface and free medium across the cortex. The endodermis 

 normally takes part ; its cells divide radially to keep pace with the new growth, forming 

 a pocket over the young root; the cells of this region become glandular, secreting 

 enzymes which digest the tissues ahead, so that penetration is not merely mechanical. 

 On reaching the exterior this digestive pocket is worn away ; though in water-plants it 

 may persist as a conspicuous glove-finger over the apex (Lemna). The xylem of the 

 young root is linked by tracheides with that of the old, at the point of origin; the 

 phloems are connected laterally, and the gap in the endodermis is made good at 

 a * 3~dot ' cell, as seen in transv. sect. Tissues of unlike origin have to be fitted together 

 or the machine would not work, and a considerable range of variation may be noted. 



Adventitious Roots arising on stem, or even leaf-structures, are similarly 

 endogenous and are initiated in the pericycle. From very embryonic tissues with im- 

 differentiated epidermis, ' bud-roots ' may take off exogenously (Water-cress). The 

 rooting of ' cuttings ' is effected by the initiation of root-centres in the meristem of 

 * wound-callus '. In many trees the factors required to initiate a stem-apex are sub- 

 stituted for those of a root, and stem-buds may thus break out endogenously from the 

 pericycle, to corne to the surface as ' suckers ' (Elm, Poplar). Adventitious roots, 

 emitted from a stem under stimulus of contact with a damp surface, may be utilized 

 for climbing, Ivy (Hedera) and Poison Ivy (Rhus toxtcodendron) ; in the event of 

 efficient water-supply these may attain a considerable size. 



The origin of lateral roots may be traced in Bean seedlings, from the first visible 

 protrusion of the young laterals to near the apex ; but owing to the vague apical 

 differentiation of the Bean root, seedlings of Zea give more detailed results. 



The development of secondary tissues may be followed on the Bean, but more 

 conveniently in the roots of : 



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