THE STEUCTURE OF THE ROOT 383 



The lateral roots in the Vascular Cryptogams do not origmate 

 in the pericycle, but in the endodermis. A cell of the latter, which 

 is opposite to a xj-leni bundle of the main root, cuts out of itself, 

 by four successive divisions, an apical cell which gives rise to 

 the tissue of the lateral root. The latter bores its way through 

 the cortex of the primary root in the way already described. 

 The pericycle takes no part in the formation except to contribute 

 the cells which connect the vessels of the lateral with those of 

 the primary root. 



In the Lyco2}odiacecE no lateral roots are produced ; the 

 main root sometimes branches dichotomously, the growing 

 point dividing into two equal ones. 



When adventitious roots are formed from the stem, they 

 usually originate in the same way as the normal lateral roots : 

 in the Cryptogams from the endodermis ; in the Phanerogams 

 from the pericycle, as already described. In a, few rare cases 

 they arise exogenously like the branches of the stem : this occurs 

 when they are developed in connection with adventitious buds. 



The so-called haustoiia, or sucking roots of parasitic 

 Phanerogams, are developed from the periblem. 



The stem and root forming together the axis of the plant, the 

 tissues of the one are continuous with those of the other. The 

 structure is, as we have seen, different in the two parts. Between 

 them there is therefore a region in which the arrangement of 

 the tissues of the one gradually changes into that of the other. 

 This is in the so-called hypocotyledonary portion of the axis, or 

 the part lying between the root and the insertion of the cotyle- 

 dons. In Dicotyledons and Gymnosperms the epidermis of the 

 stem is continuous with the piliferous layer, which we have seen 

 is generally the inner layer of the dermatogen, the outer layer of 

 which forms the only true epidermal portion of the root, the root- 

 cap. In Monocotyledons the epidermis of the stem is at first 

 continuous with the epidermis of the root. The latter, however, 

 soon separates and exfoliates with the outer layers of the root- 

 cap, leaving the external layer of the periblem naked. The latter 

 becomes the piliferous layer. 



The endodermis and pericycle of the two regions pass directly 

 into each other, and the intermediate cortical tissues are similarly 

 continuous. 



The vascular bundles of the root become continuous with 

 those of the stem, but the two constituents change their posi- 

 tions considerably. 



In the commonest case each of the xylem bundles of the root 



