THE PRIMARY MERISTEMS 95 



meristem depends upon the structure of the growing-point, Where 

 a single apical cell is present, or where the initials are juxtaposed, the 

 protodermal cells become cut off sooner or later, according to circum- 

 stances from the primary segments by periclinal walls. In the root- 

 apex of Ferns, for example, the protoderm is differentiated, in this 

 manner, at a very early stage, immediately after the appearance of 

 " sextant- walls " in the root-segments cut off' from the apical cells. 

 Each sextant divides by a periclinal wall into an inner and an outer 

 cell ; the latter at once again divides by another periclinal wall, and the 

 outer of the two resulting daughter-cells becomes an element of the 

 protoderm. In the stem-apex of Erprisrtum, on the contrary, the pro- 

 toderm separates at a much lat.er stage, after the primary segments have 

 undergone numerous radial, periclinal and anticlinal divisions. Where 

 the apex contains several superimposed initials it is, as has already 

 been explained, very usual indeed for the product of the uppermost 

 initial, or group of initials, to remain permanently single-layered, in 

 which case it becomes directly converted into the protoderm. In these 

 special circumstances the protoderm constitutes an independent super- 

 ficial histogenetic layer or dermatogen. It is, however, hardly necessary 

 to insist that the author's " protoderm " embodies a much wider con- 

 ception than Hanstein's "dermatogen"; the former term was introduced 

 without any reference to the structure of the growing-point, whereas 

 the latter connotes a special type of apical structure. 



The boundary between the superficial protodermal layer and the 

 more deeply situated meristematic layers is not equally well marked in 

 all cases. Where the protoderm is late in appearing, the radial diameter 

 of its component elements is variable, and its inner boundary is conse- 

 quently somewhat ill-defined. Where, on the contrary, the protodermal 

 layer is continued right over the apex as the dermatogen, its inner 

 (periclinal) walls, when seen in transverse or longitudinal section, 

 usually present the appearance of a very regular zigzag line. 



Once the protoderm is differentiated, its subsequent divisions are 

 usually all radial (anticlinal). Less frequently, divisions also take 

 place in the tangential (periclinal) direction, with the result that 

 several protodermal layers are formed; this latter condition is found 

 where the adult epidermis is many-layered, as w 7 ell as in connection 

 with the development of certain special tissues or organs. 



2. The second type of primary meristem, or procambium, consists 

 of narrow, prosenchymatous meristematic cells (Fig. 22c); it is con- 

 cerned with the production of the bulk of the vascular ami fibrous 

 " strands " of the adult plant, But just as the protoderm may form 

 other structures besides the dermal tissues, so the procambiuin need 

 not always give rise to bundles alone ; it may, on the contrary, after 



