146 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[30. 



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

 elongated cells (p p) constituting the plerome, and terminating 

 in one or more initial cells ; on tracing 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 said to be monostelic. In some cases, 

 however, more than one plerome- strand is present at the growing- 

 point, and the stem consequently contains more than one stele 

 {e.g. Gunnera), and is said to be polystelic. 



Between the dermatogen externally and the plerome internally 

 is a layer less frequently several layers of cells constituting the 

 periblem ; the cells of these layers undergo divisions both 

 anticlinally and periclinally, so that both the area of each layer 



and the number of the 

 layers are increased. 

 On tracing the periblem 

 backwards into the older 

 parts, it is found to be 

 continuous with the 

 extra-stelar fundamental 

 tissue. 



The growing-point of 

 the root (Fig. Ill) of 

 one of these plants es- 

 sentially resembles that 

 of the stem in its struc- 

 ture ; the small -celled 

 meristem is differenti- 

 ated, at least primarily, 

 into dermatogen, ple- 

 rome, and periblem. But there is this distinctive peculiarity 

 about the dermatogen of the root, that its cells undergo division, 

 not anticlinally only, as in the stem, but periclinally also, so that 

 the epidermis of the root is many-layered (except in Hydrocharis 

 and Lemna, where it remains a single layer). This many-layered 

 epidermis, however, is gradually exfoliated as the parts grow 

 older, and persists only at the apex constituting the root-cap 

 (see p. 6). The only other important fact to be noticed at present 

 is that the root has only a single stele, that is, it is always mono- 

 stelic. 

 The foregoing description is applicable to all growing-points with a small- 



FJG. 110. Median longitudinal section of the grow- 

 ing-point of the stem of Hippuris vulgarig. The 

 growing-point consists of a small-celled meristem, 

 differentiated into dermatogen d, plerome pp, arid 

 periblem consisting of the five layers of cells between 

 the plerome and the dermatogen ; I rudiment of a 

 leaf. (After de Bary ; x225.) 



