OWING-POINTS OF STEMS AND ROOTS 149 



always completely enclosed and protected by young leaves 

 (Fig. 76) and consists of a dome-shaped mass of meristem, 

 from which are derived all the various tissues already studied in 

 the mature stem and leaf. The cells forming the meristem, are 

 approximately uniform in size and form : they possess thin walls 

 and are rich in protoplasm. 



In a favourable longitudinal section through the growing-point 

 three distinct strata are often visible (Figs. 76 and 77). Covering 

 the apex is a single layer {d) termed the dermatogen which divides 

 only by walls at right angles to the surface and gives rise to the 

 epidermis of the plant. 



Beneath the dermatogen comes the periblem (a) from which 

 the cortex is derived. At the extreme apex it may be only one 

 cell thick, but in the older parts division takes place in several 

 directions and a many-layered stratum is produced. 



Occupying the centre is a solid mass of meristem termed the 

 plerome (b) : from it the vascular cylinder is developed within 

 which at a short distance from the apex the differentiation of the 

 vascular bundles begins to appear. 



The leaves of the plant are first seen as slight projections (/) 

 on the surface of the growing-point ; the tissues taking part in 

 their formation are the dermatogen and a portion of the periblem. 



The branches which arise in the axils of the leaves are also 

 developed from the dermatogen and periblem ; the plerome 

 is not concerned in the production of either leaves or 

 branches. 



(ii) Growing-point of the root. — The apex of a root differs very 

 considerably from that of a stem. The delicate meristem in the 

 latter always exists within a bud and is protected from external 

 injurious influences by the rudimentary leaves which curve 

 round it. 



Roots, however, produce no leaves, but the tender cells of the 

 meristem at the apex of each are protected by a covering of 

 cells termed the root-cap. Moreover, as fast as the exterior of 



