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Endo dermis in the Stem of Hippuris . 
The advantages of polychrome methylene blue are twofold ; it is a very 
quick and easily controlled stain, and it differentiates well. The walls 
of the cells lining the air-passages stain differently from the others, and 
are thereby easily distinguished even in longitudinal sections. 
The structure of the apex of the stem as seen in median longitudinal 
section (Fig. i) exhibits an arrangement of cell-layers which follows dia- 
grammatically the scheme set forth for stem apices in general. The outside 
is clothed by a single layer of regularly arranged cells, which constitutes 
a typical dermatogen. Within this, in the specimen figured, are five regular 
layers of cells forming the periblem. The cells of these differ little in form 
from those of the dermatogen. Within these, the cells of the plerome are 
more irregularly arranged, owing to the fact that they divide by both 
periclinal and anticlinal walls. 
Fig. i. Longitudinal section of tip of stem of Hippuris vulgaris, d, dermatogen ; p, periblem ; 
pP plerome. 
There is considerable variation in the size of the stem apices, and 
the number of periblem caps in the material examined varies from three to 
six. The general arrangement in other respects is quite consistent. 
On turning to the transverse series and examining the first section 
(Fig. d) it is a simple matter to interpret the layers of cells in terms of the 
longitudinal section. In some cases there is more difference between the 
size of cells of the periblem and those of the plerome than appears in 
the specimen figured. This difference becomes more pronounced as the 
sections are traced further down the stem. The cells of the plerome at an 
early stage divide rapidly by walls in all directions, thus producing a large 
number of small cells. The method of division is fairly regular. One cell 
divides into two, and these again divide independently by walls nearly 
cit right angles, thus forming groups of four cells. These groups frequently 
