BUD OF HIPPURIS. 



211 



upon this, and to cover it with a second cover-glass. We can 

 then, if needed, turn over the section together with the cover- 

 glasses, and so examine it on both sides ; we must, however, 

 take care that no fluid gets between the under cover-glass and 

 the object-slide. For use in this way we can get thin perforated 

 object-slides, upon which latter the lower cover-glass can first of 

 all be cemented, and thus the whole slide can be inverted. 



With pretty strong magnification, we settle, in the first place 

 (compare Fig. 82), that there 

 is a thoroughly definite ar- 

 rangement of the cells in the 

 " meristem " of the growing 

 apex. There are cap-like layers 

 of cells, the separating walls 

 of which form a series of 

 confocal parabolae. The outer- 

 most layer of cells, which 

 covers the growing apex and 

 also extends as a simple cell- 

 layer over the leaf protuber- 

 ances, is the initial layer of 

 the epidermis the Derma- 

 togen (d). Under this lie four FlG - 82.-Longitudinal section through the 



growing apex ot Hippv.ns mugaris. a, aer- 

 Ol m more undifferentiated layers matpgen ; pr, periblem ; pi, plerome ; /, 



of cells (meristem layers), r 



which appertain to the Periblem (pr), and from which the primary 

 cortex of the stem proceeds. Lastly, we find a central cylinder, 

 which tapers conically upwards, ending with usually one cell, and 

 out of which, as can be demonstrated lower down in the section, 

 the central cylinder of the stem is formed. This tissue we desig- 

 nate the Plerome (pi}. Epidermis, primary cortex, and central 

 cylinder, have, therefore, in Hippuris their own special " histo- 

 gens " or histogenic layers. 



Not, however, in all growing apices of Phanerogams is the 

 separation of the "histogens" so sharply marked as in this case. 

 In many Gymnosperms (Abietineae, Cycads) a clear separation 

 between dermatogen and periblem does not exist, and often the 

 periblem is not clearly defined from the plerome. In Angio- 

 sperms the dermatogen is always clearly defined, but a clear 

 limit between periblem and plerome is often not present. The 

 question is not therefore in general one of a differentiation of the 



m 



