i68 
MORPHOLOGY OF MEMBERS. 
indicated by the splitting of several cells of the pericambium of the mother-root by- 
tangential walls, so that it is divided into two layers (Fig. 125, A). 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 h). The 
inner layer of cells {A, n n), 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 is 
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 (D) ; 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 par'v'i/Iora 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 Hke 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 NUus-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 Nägeli 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 Algse 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 Fucacese and Laminarieae, but even in the unicellular Faucheria and Caulerpa. 
In confirmation of the Theory of Descent referred to at the conclusion of this work 
(see Book III. Chap. 7), it is of great importance to know that members differing 
