794 A TEXTBOOK OF THEORETICAL BOTANY 



Meristems. 



The meristem of the root is very different from that of the stem. The 

 outgrowth of leaves from the latter produces more complex conditions, 

 which we shall have to consider later, and the morphology of the meristem 

 is much more varied. The root apex forms a rounded cone, the tip of which 

 consists of the root cap or calyptra. The actual meristem lies beneath this 

 covering and so at some little distance from the surface. 



No single apical cell can be found ; instead there is a cluster of initial 

 cells, the fundamental meristem, from which radiate the concentric zones of 

 cells, which form the permanent tissues and are easily recognized in longi- 

 tudinal sections. The group of initials usually consists of several super- 

 imposed layers of cells, but this is very variable. It constitutes the true 

 meristem and its cells are closely coherent, with no air spaces between them 

 and they contain no vacuoles. They have a twelve-sided or fourteen-sided 

 shape, which is that formed when plastic spheres are pressed together in a 

 closed space. This differentiates them clearly from the developing, but 

 still meristematic young tissues, whose cells are regularly rectangular. 



In 1868 Hanstein put forward a theory of germinal layers, or histogens, 

 at the apex of the stem and root. He distinguished a central core or plerome, 

 consisting of more or less elongate cells, around which lies a shell of periblem 

 consisting of several concentric layers of rectangular cells. The outer^Tayer 

 or dermatogen is normally only one cell thick and is distinguished by 

 the uniformly anticlinal division of its cells {i.e., the division planes are 

 perpendicular to the outer surface). 



This is in fact no more than a statement of what observation shows us, 

 and such layering necessarily arises when the initial group has at least three 

 layers, superimposed vertically. Hanstein's theoretical limitation of these 

 layers as " germinal," was influenced by the prevailing studies on animal 

 embryos. He considered that the plerome produced the stele, the periblem 

 gave rise to the cortex and the dermatogen produced the epidermis. Careful 

 study has shown that these limitations are not generally fulfilled. As we 

 shall see, the histogen theory has been dropped in the interpretation of the 

 stem apex. It has still, however, a descriptive value in the root apex (see 

 Fig. 790), especially in connection with the origin of the root cap. The 

 surface layer of the root is not, however, an epidermis and the outer germinal 

 layer is not continuous over the apex as the dermatogen is in the stem, so 

 that it is better to adopt for it the term protoderm. The plerome cells cease 

 dividing and begin elongating earlier than those of the periblem, and the 

 differentiating zones of the periblem and plerome are fairly easily dis- 

 tinguishable (Fig. 791). 



The Root Cap. 



The root cap is usually described as arising from a special histogen layer, 

 the calyptrogen. This is true in many cases, but not in all. The most 



