146 



EXTERNAL CONFORMATION OF PLANTS. 



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 body of the root high up, and is only fixed below to the 

 apex of the root. 



(d) We have already, in sect. 19, touched on the manner in which the primary 

 meristem of the apex of the root is differentiated in Angiosperms into three layers, the 

 Dermatogen (primordial epidermis), the Periblem (primordial cortex), and the interior 

 tissue or Plerome. In roots which remain slender, like those of Cryptogams and of many 

 Phanerogams, the whole of the plerome is transformed into an axial fibro-vascular cylinder, 

 in which two, three, or more vascular bundles arise. The vessels are formed first of all 



near the periphery of the bundle, at two, three, or 

 more points of the section, and afterwards further 

 towards the interior, until the rows of vessels meet in 

 the middle, and form a diametral row, or a star of 

 three, four, or six rays\ The lateral roots appear on 

 the outside of the bundle in front of the primary ves- 

 sels. Between the vascular bundles bast-bundles are 

 formed (although not always) somewhat later in the 

 peripheral gaps of the axial fibro-vascular cylinder. 

 On originally thicker roots of Phanerogams the axial 

 cylinder or plerome becomes again differentiated ; its 

 central portion becomes parenchymatous and forms 

 a pith (Fig. 116, m), while vascular and bast-bundles 

 arise only in the peripheral portion. When a root 

 is able to increase subsequently in thickness, like 

 the napiform primary roots of Dicotyledons, there 

 is formed in the thickening-ring x (Fig. 116) be- 

 tween the vascular bundles g and on the inside of 

 the bast-bundle b, a secondary meristem, a true 

 cambium, which on its part behaves exactly like the 

 cambium of a stem capable of subsequent increase 

 of thickness ; it produces inwardly in a centrifugal di- 

 rection xylem, outwardly bast, especially phloem. Fig. 

 116 shows at ^ the transverse section of the primary 

 root of a bean before the increase in thickness has 

 begun ; and at B after the growth has continued 

 for some weeks; between the thin pith m and the 

 primary cortex pr a four-rayed woody substance has 

 been formed; the four intermediate ' medullary rays' 

 correspond to the original woody bundles gg which have not continued to develope 

 centrifugally ; the primary bast-bundles are still visible in B, i? ; but in addition, the 

 cambium has produced a layer of phloem with secondary bast-fibres /?'. The strong 

 primary root of the maize also produces, as has already been shown in Fig. 104, sect. 19, 

 by the differentiation of the plerome, a pith-like substance m, surrounded by a fibro-vas- 

 cular hollow cylinder .v, in which are formed vessels and elongated wood-cells. Bast- 

 cells are here not so clearly visible as in Phaseolus, or not at all. Fig. 117 shows the 

 transverse section of the same root, somewhat higher than the longitudinal section in 

 Fig. 112. No subsequent increase in thickness takes place in this case, nor is such active 

 cambium formed in the fibro-vascular bundle as in Phaseolus. These are only some of the 



^" S 



k 



Fig. 116.— Transverse section of the primary 

 root of Phaseolus multifloriis (slightly mag- 

 nified). A a few centimeters above the apex 

 of the root; B higher up on a much older 

 root ; pr primary cortex ; ni pith ; x thic ken- 

 ing ring ; gggg primary vascular bundles ; 

 bbbb primary bast bundles; b' secondary 

 bast ; k cork. 



In the thin embryo-roots of Triticum and other Grasses, an apparent central (axial) vessel 

 is formed. 



