160 BOTANY PART i 



periblem, and calyptrogen are not marked out as distinct regions. 

 In roots, as in stems, the plerome cylinder (pi) almost always 

 terminates in special initial cells. 



At a short distance below the growing point the embryonic tissue 

 loses its meristematic character, and becomes transformed into the 

 differentiated body of the plant. As a general rule, in plants with 

 an epidermis, primary cortex, and central cylinder, the epidermis 

 is developed from the dermatogen ; the primary cortex from the 

 periblem ; the central cylinder from the plerome. This differentiation 

 of the tissues does not take place in all cases; an already determined 

 separation of the several tissues must not be assumed to extend to 

 the embryonic tissue, the peculiar cell arrangement of which is due 

 rather to physical causes. The vascular bundles must pass through 

 the periblem in order to reach the leaves. The periblem is therefore 

 capable of producing, not only the primary cortex, but also the 

 vascular bundles and accompanying tissues of the central cylinder. 

 The terms dermatogen, periblem, and plerome are employed merely 

 for convenience to designate certain cell layers, and are not to be 

 regarded as significant of any peculiar histogenetic or tissue-forming 

 ability. The external layer from which the epidermis develops usually 

 remains a single cell layer. The rudiments of the still undeveloped 

 vascular bundles soon appear in the central cylinder as procambium 

 strands ; while the endodermis of roots is derived at an early stage 

 from the innermost layer of the cortex. 



In stems with apical cells the RUDIMENTS OF NEW LEAVES and 

 SHOOTS are developed from single peripheral cells, or cell groups of 

 the vegetative cone (Fig. 164). In such cases, not only the new 

 shoots, but even the leaves, usually begin their development with an 

 apical cell. The apical cells of the leaves, however, soon disappear, 

 and further growth proceeds along their whole margin. 



In those fern leaves which are able to form terminal buds (p. 31) GOKBEL ( 155 ) 

 has observed the direct transition of the two-sided apical cell of the leaf tip to the 

 three-sided pyramidal apical cell of the young shoot ; this takes place by the 

 origin of a suitably placed wall in the apical cell of the leaf. 



In a stem with no apical cell (Figs. 17, 166), the rudiments of 

 the leaves and new shoots first appear as small protuberances, the 

 formation of which is generally initiated by the periclinal division of 

 a group of periblem cells ; while the cells of the overlying dermatogen 

 continue to divide by walls at right angles to the surface. In the 

 case of new shoots developing at some distance from the growing 

 point of the parent stem, the cells from which they are destined 

 to arise retain for that purpose their original embryonic character. 

 The formation of the buds on the rapidly growing shoots of bushes 

 and trees in the spring may be postponed, so that the rudimentary 

 lateral shoots first appear in the axils of the eighth or even the tenth 



