AND HISTOLOGY OF PIPER BETLE. 369 
subsidiary cells is cut off from the cell which forms the mother-cell of the 
guard-cells. The preparatory divisions take place usually in four radii. 
The stomatal type therefore approaches that in the Cruciferce *, in which, 
too, there is a subsidiary ring similarly developed, but it is of three cells 
instead of four. The outer ring, when present, is contributed by the sur- 
rounding cells. The guard-cells are crescentic as usual, and measure 24 w in 
length and 7 in breadth when the stoma is moderately open. Thev are on 
а level with the rest of the epidermal cells. They do not show any special 
thickening or protrusions, 
The lower hy podermal layer, as already stated, is only one cell thick. This 
layer also, like the upper one, is derived from the dermatogen. Its cells 
cease to divide earlier than those of the epidermis, so that in a section of 
a young leaf there are to be seen about twice as many epidermal cells as 
hypodermal ones. On the other hand, the number of cells in both the upper 
hypodermal layers corresponds with that in the upper epidermis at the same 
stage (Pl. 19. fig. 35). The cells of the lower hypoderma about this time 
look columnar in a transverse section, their height being about as much as 
that of the two layers on the other side taken together, or twice to thrice as 
much as that of the epidermal cells lying in contact with it. At this stage 
there are no intercellular spaces to be seen. The secretory cells, however, 
make а very early appearance and acquire full size almost from the beginning. 
The fully developed leaf shows large intercellular spaces or respiratory 
chambers in this lower hypodermal layer. They are of all sizes and shapes. 
In surface-view the most prominent ones are circular and measure about 
65 across. They generally lie over the stomata. Wherever there are 
secretory cells the ordinary cells seem to radiate towards them; the secretory 
cell looks like a hub round which there are about six somewhat wed ge-like 
radiating cells to form the spokes. 
Mesophyll.—The mesophyll is of the bifacial type. The palisade-tissue is 
irregularly stratified. The maximum number of strata is two. Whenever 
the two strata are developed there is an encroachment on the hypodermal 
layer, the thickness of which is reduced to one cell thereat (P1. 18. fig. 22). 
The double strata are short and few. The greater length of a section 
shows only a single layer or stratum of palisade-cells. The cells are roughly 
cylindrical in outline, and measure on an average 40 р by 12 p; they are 
close-set. The only intercellular spaces present are those that a bundle 
of cylinders would naturally enclose (РІ. 18. fig. 16). When two strata are 
present, the height of the double layer is roughly twice that of the single 
layer, and the cells of the two layers are of equal height. The regularity of 
the palisadé-layer may be broken by the presence of the ubiquitous secretory 
cells. The cell-walls are free from all markings. 
The spongy tissue is made up of irregularly rounded cells. The smaller 
* Solereder, Syst. Anat. Dicot., Engl. Trans. p. 59. 
