HAIRS (TRICHOMES). 



H5 



segments, from which the cap arises, as has already been shown in sect. 19, Fig. 102, 

 p. 125. The roots of Lycopodiaceae do not produce any lateral roots, they branch 

 instead dichotomously at the apex (Fig. 130, p. 161). 



In Phanerogams, the commencement of a lateral root is indicated by the splitting of 

 several cells of the pericambium of the mother-root by tangential walls, so that the 

 pericambium is divided there into two layers (Fig. 115, ^). The outer layer is im- 

 mediately formed into 

 dermatogen (d), which 

 afterwards forms the 

 layers of the root-cap 

 by tangential divisions; 

 while the outer layer of 

 cells which proceeds 

 from the young derma- 

 togen always constitutes 

 a layer of the root-cap 

 (C/j). Theinneriayerof 

 cells, resulting from the 

 splitting of the pericam- 

 bium (^, n n), which 

 faces the vessels of the 

 bundles of the mother- 

 root, then also splits 

 again into two layers 

 (B) ; and further longitu- 

 dinal and transverse di- 

 visions follow, by which 

 the primary meristcm 

 of the young root is 

 formed. This soon di- 

 vides into three parts; a 

 basal part by which the 

 young root remains in 

 connexion with the vas- 

 cular bundle of the 

 mother- root (D, m m), 

 and an anterior mass of 

 tissue which becomes 



differentiated into pericambium and plerome (D, pp). While the young root lengthens 

 in a direction transverse to the axis of the mother-root, somewhat obliquely downwards, 

 it compresses the cortical tissue (D) ; the innermost layer of cortex {A-D, r) resists 

 disorganisation longest, and, at least at first, follows the growth of the young root, sur- 

 rounding it in a sheath-like manner till it is destroyed. Finally, therefore, the young 

 root lengthens and its apex protrudes through the cortical tissue of the mother-root. 



In stem-formations lateral roots arise either from the interfascicular cambium 

 (e. g. in Impatiens parinflora immediately above the soil in the primary stem), or from 

 the outermost phloem-layer of the fibro-vascular bundles, which is more commonly the 

 case. These layers of tissue then behave like the pericambium of a mother-root {e. g. 

 Veronica Beccabunga, Lysimachia nummiilaria, the ivy, according to Reinke). 



(c) Whilst the formation of the root-cap, as has already been shown in sect. 19, 

 continues from the apex of the root, its outermost layers of tissue pass over into 

 permanent tissue ; the cells retain simple forms, but their walls become thicker, 

 and swell up in the outermost cell-layers of the cap, become gelatinous, and thus 

 cause the apex of the root to appear viscid ; finally they die and become detached. 



L 



Fig. 115.— Mode of formation of the lateral roots in a mother-root of Trapa 7iatans {after 

 Reinke). A the pericambium (ir) bounded by the innermost cortical layer splits into der- 

 matogen (if) and an inner layer n, which in B is already ajjain divided. C young root enclosed 

 in the tissue of the mother-root ; R r cortex of the latter; tt the pericambium of the mother- 

 root from which the secondary root has been formed ; h the first layer of its root-cap, d its 

 dermatogen. D secondary root in a further stage of development, surrounded only by the 

 innermost cortical layer r of the mother-root; // its periblem, in the middle of the ple- 

 rome ; fft tn the tissue that supplies the connexion with the mother-root. 



