102 PART II. ANATOMY AND HISTOLOGY. [ 29 



In the stem, a growing-point of this kind usually presents the 

 appearance shown in Fig. 83. It consists, in the first place, of a 

 well-defined superficial layer which, on being traced backwards, 

 is seen to be continuous with the primary tegumentary tissue 

 (epidermis) of the older parts ; this layer is, in fact, the embryonic 

 epidermis or dermatogen (d} ; it is quite distinct, morphologically, 

 from the subjacent cells, and is characterized by the fact that its 

 cells only undergo division in planes perpendicular to the surface 

 (anticlinal), and not in any plane parallel to the surface (periclinal). 

 Consequently, whilst the dermatogen increases in area, so as to 

 keep pace with the growing tissues within, it does not become 

 many-layered, but remains a single layer of cells. 



In the middle of the growing-point is a solid mass of somewhat 



elongated cells (pp) con- 

 stituting the plerome, and 

 terminating in one or 

 more initial cells ; on trac- 

 ing this backwards into 

 the older part of the stem 

 it is found to give rise to 

 a single axial cylinder of 

 tissue, the stele, in which 

 the vascular tissue is de- 

 veloped. Such a stem is 

 /* P said to be monostelic : 



Fxo. 83.-Medianlongitudinal section of the grow- 

 ing-point of the stem .of Hippuris vvlgaris. The point of this structure 

 growing-point census of a small-celled meristem & ^^ without excep . 

 differentiated into dermatogen a, plerome p p, and ^ " 

 periblem consisting of the five layers of cells between tion, monostelic. 

 the plerome and the dermatogen ; I rudiment of a Between the dermato- 



leaf. (After de Bary : x225.) 



gen externally and the 



plerome internally, is a layer less frequently several layers of 

 cells constituting the periblem ; below the apex the cells undergo 

 divisions both anticlinally and periclinally, so that both the area 

 and the number of the layers are increased. On tracing the peri- 

 blem backwards into the older parts, it is found to be continuous 

 with the ground-tissue which, in monostelic members, is known as 

 the primary cortex. 



The growing-point of the root (Fig. 84) of one of these plants 

 essentially resembles that of the stem in its structure ; the small- 

 celled meristem is differentiated, at least primarily, into dermatogen, 



