STRUCTURE OF THE ROOT 91 



lateral root (Fig. 65, b). By reason of its deep-seated origin the tissues 

 of the lateral root are intimately connected with the tissues of the main 

 root. At its tip it pierces the cortex of the parent root by a process of 

 digestion, through the activity of cells of the endodermis (end.), though 

 in some cases cells of the cortex also act in the same way. They form a 

 glandular digestive sac which softens the cells outside, so that they yield 

 to the growing rootlet within. The substances of the digested cells 

 are absorbed by the rootlet as it forces its way out from the parent. 



The details of division of cells in the formation of a rootlet vary in different 

 cases. It is here illustrated for the type usual in Dicotyledons. The cells 

 of the pericycle divide by periclinal walls, so as to form several layers, and by 

 anticlinal walls, so that each layer consists of numerous cells. The innermost 

 layer forms the plerome of the young root, giving rise to its central stele (pier.). 

 The next outer layer is the periblem, giving rise to the cortex (peribl.) ; while 

 the third outermost layer gives rise on the one hand to the root-cap (calyp.), 

 and on the other to the piliferous layer {pilif.) ', the two having a joint origin, 

 as already explained in the case of the Buckwheat. The consequences of this 

 mode of origin are that the vascular tissues of the rootlet connect directly 

 with the vascular tissues of the main root. This is essential to the effective 

 transfer of materials. But the cortex, piliferous layer, and root-cap of the 

 lateral root are all distinct in origin from those of the parent. Physiologically 

 this discontinuity is not a matter of importance. 



The individual root thus growing indefinitely at its apex, and bearing 

 an increasing number of lateral roots, is subject to increasing demands 

 upon it, as a means of transit from the distal absorbing region to its 

 attachment at the base of the plant. In Monocotyledons (Fig. 58), 

 and in some Dicotyledons (Fig. 59) there is no special development 

 of tissues to meet this, but there is usually an increase in the number 

 of adventitious roots, as in Palms, Maize, Onion, etc. In most 

 Dicotyledons, and especially in those that are woody, there is a 

 process of secondary thickening by means of a cambium, analogous to 

 that seen in their stems. Cell-divisions appear in the conjunctive paren- 

 chyma lying internally to the several groups of phloem (Fig. 66, a). 

 Arcs of cambial activity are thus formed, which soon spread to the 

 tissue of the pericycle lying peripherally to the protoxylem (Fig. 66, b). 

 The several arcs are thus linked together to a continuous band, in the 

 form of a corrugated cylinder. As in the stem, this cambium produces 

 secondary wood internally, and secondary bast externally, to an 

 indefinite degree. But at first the cells lying peripherally to the 

 protoxylem form only parenchyma, so that a broad medullary ray 

 appears externally to each group of the primary wood. In the 

 secondary wood additional medullary rays and annual rings may 



