86 



BOTANY 



PART 



place of this a number of equivalent meristematic cells, which often 

 form regular concentric layers (Fig. 102), are met with. 



The outermost layer of cells which covers the growing point and also the 

 developing leaves is termed the DERMATOGEN (d) because it gives rise to the 

 epidermis ; it usually divides by anticlinal walls only. The cells in which the 

 central cylinder of the stem ends at the apex form the PLEROME (pi), while the layers 

 between this and the dermatogen constitute the PERIBLEM (pr). The limit between 

 the periblem and plerome is often indistinct. The leaves and lateral branches arise 

 as multicellular projections (Fig. 102), which come about by local increase in 

 number of periblem cells, while the dermatogen undergoes anticlinal divisions only 



FIG. 100. Median longitudinal section of the vegetative cone of Equisetum arvense. 

 Explanation in the text, (x 240. After STRASBUROER.) 



and keeps pace with the enlargement. In the case of the origin of leaves only the 

 dermatogen and periblem are concerned ; in that of the lateral branches the 

 plerome also ( 55 ). 



Since, the rule that the new cell walls intersect at right angles holds for 

 growing points, the system of cell walls as seen in longitudinal sections often 

 forms a strikingly symmetrical figure, the periclinal as well as the anticlinal 

 walls forming systems of confocal parabolas (Fig. 266). The elements of the one 

 system cut those of the other nearly at right angles (SACHS). In transverse sections 

 of such growing points the periclinal walls form concentric circles. 



BUD. The formation of new members at the growing point is 

 followed by their increase in size and differentiation. This applies in 

 the first place to the young leaves, the growth of which exceeds that 

 of the stem apex and is most marked on their lower surfaces. As a 



