AND HISTOLOGY OF PIPER BETLE. 37E 
systematic histology cannot be estimated at present, for it has not been 
systematically worked out. 
Leaf- Margin—The margin, which is always more or less strongly 
recurved, is characterised by the absence from it of the assimilatory tissue. 
It is the line of union of the hypodermas of the two surfaces. The cells in 
this region have somewhat thickened walls, but there is no differentiation of 
collenchyma or selerenehyma, as is the case with many leaves having 
recurved margins. The assimilatory tissue abutting on the margin is 
particularly rich in secretory cells ; it also contains a marginal vein running 
through it. 
Petiole (Pl. 19. fig. 33).—A section of the petiole through tho charac- 
teristic region *, i. e. at the point neurest the blade, has the following 
characters. The outline is very variable. The following generalisation, 
however, ean be made. The lower border is convex. It may or may not be 
rariously lobed. The upper border is either flat or slightly notched, or 
deeply grooved along the middle. 
The epidermis is of the same character as that of the stem to be described 
below. 
Within the epidermis there is a very narrow band, only two or four cells. 
thick, of parenchyma containing chloroplasts. On it abuts the collenchyma- 
zone, which is not a continuous girdle, but is divided into a variable number 
of ares. The usual number is eight or nine. These ares are separated by 
narrow bridges of parenchyma half a dozen cells broad. Within. the 
collenehyma lies a mass of parenchyma, which encloses intercellular spaces, 
Secretory cells permeate the parenchyma throughout ; they are particularly 
abundant in the immediate neighbourhood of the collenchyma-ares, both 
within and without them. The centre of the section is occupied by a single 
large mucilage-canal. Round it are ar ranged in a crescent, which may 
close up into a circle, the vascular bundles. They are graded in size. The 
three largest lie on the underside, 7. e., between the lower convex border and 
the centre, forming a small open аге. The are is continued laterally upwards 
by two more bundles on either side, which are smaller than the three bottom 
ones. The are may or may not be completed into a circle by two or three 
very small bundles lying on the upper side, 7. e., between the upper border 
and the centre. Between the seven major bundles there may or may not be 
interealated tiny bundles, one between each two. These may either fall 
in the same circle as that of the main bandles or they may lie outwards so 
as to form an independent outer circle of their own; or, finally, some may 
be in one position and some in the other. 
Regarding the orientation of the bundles, the xylem points towards the 
centre and the phloem towards the circumference. This rule, however, is 
* See Petit’s terminology quoted in Solereder's Syst. Anat. Dicot., Engl. Trans. p. 1094. 
