l68 MORPHOLOGY OF MEMBERS. 



indicated by the splitting of several cells of the pcricambium of the mother-root by 

 tangential walls, so that it is divided into two layers (Fig. 125, ^). The outer layer 

 is immediately constituted into dermatogen (d), which afterwards forms the layers of 

 the root-cap by tangential divisions ; since each outer layer of cells which results from 

 the successive layers of the dermatogen constitutes a layer of the root-cap (C^). The 

 inner layer of cells (^, « «), which faces the vessels of the vascular bundle of the 

 mother-root, then also splits again into two layers (B) ; and further longitudinal and 

 transverse divisions follow, by which the primary meristem of the young root is formed. 

 This soon divides into periblem and plerome, as may be clearly seen in D, where p p \s 

 the periblem, and m m the basal portion of the plerome, by which a union is effected 

 with the fibro-vascular cylinder of the mother-root. While the young root lengthens 

 somewhat obliquely to the axis of the mother-root and downwards, it compresses the 

 cortical tissue (i)) ; the plerome-sheath {A-D, r) resists disorganisation longest, and, at 

 least at first, follows the growth of the young root, surrounding it with a sheath until 

 it is destroyed. Finally the young root lengthens and its apex protrudes through the 

 cortical tissue of the mother-root. 



(c) In stems lateral roots arise either from the interfascicular cambium {e.g. in Im- 

 patiens paw'iflora 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 primary root, as in Veronica Becca- 

 bunga, Lysimachia nummularia, or the ivy, according to Reinke. 



(d) While the formation of the root-cap, as has already been shown in Sect. 19, 

 is proceeding at the apex of the root, its outermost layers pass over into per- 

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

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

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

 aerial and underground roots the root-cap is closely attached to the substance of 

 the root by its oldest layers, which generally extend backwards ; in the roots of Lem- 

 naceae, Stratiotes, and some other plants, which float on the water, it forms a loose 

 sheath which envelopes the substance of the root high up, and is only fixed below to its 

 apex. 



(e) Roots are generally clearly distinguished, by the characteristics mentioned 

 above, from leaf-bearing shoots ; there occur, however, a few transitional forms 

 which show that roots can become directly transformed into leafy shoots, as in 

 Neottia Nidus-a'vis, where (according to Reichenbach, Irmisch, Prillieux, and Hof- 

 meister) older lateral roots of the stem throw off their root-caps and form leaves 

 beneath the apex. On the other hand, leaf-bearing shoots cease to produce leaves, as 

 in many Hymenophyllaceae, and, according to Mettenius, form root-hairs, and assume 

 the habit of true roots (whether they actually form a root-cap is doubtful) ; in these 

 species true roots are wanting. In Psilotum triquetrum Nageli and Leitgeb have shown 

 that the apparent roots are only underground shoots, on which more or less evident 

 traces of leaf-formation may be recognised ; they resemble true roots in function and 

 in the mode of formation of their tissue, but have no root-cap, and, when they come 

 above ground, grow in the manner of ordinary leafy shoots. In Selaginelleae also, the 

 same investigators have shown the presence of leafless shoots (rhizophores) which 

 grow downwards, and do not form root-caps until they touch the ground (see Book II, 

 Lycopodiaceae). 



We thus see that transitional structures between roots and leafy shoots are found 

 even in highly differentiated plants. But even in Algae the thallus is often fixed to 

 its substratum by organs of attachment, which may be compared with roots in their 

 habit and in many functional properties ; and this occurs not only in the case of the 

 large Fucaceae and Laminarieae, but even in the unicellular Vaucheria and Caulerpa. 



In confirmation of the Theory of Descent referred to at the conclusion of this work 

 (see Bpok III. Chap. 7), it is of great importance to know that members differing 



