THE EPIDERMAL TISSUE. 
the ovary {e.g. in Ricinus). They are most numerous where an active interchange 
of gases takes place between the plant and the external air ; for, considered physiologi- 
cally, they are nothing but the mouths of the intercellular spaces of the inner tissue 
which open externally between the epidermal cells ; this is however always preceded 
by a peculiar development in a young cell of the epidermis. Since the stomata do not 
arise till a late stage in the development of the internodes and leaves, or even after their 
expansion, their arrangement is partially dependent on the already elongated form of the 
epidermal cells ; if these are greatly elongated in one direction and arranged in rows 
(as in Equisetum and the stem and leaves of many Monocotyledons and Pinns)., the 
stomata are also arranged in longitudinal rows, the cleft lying in the direction of 
the axis of growth, the guard-cells right and left ; if the epidermal cells are irregular 
on a superficial view, curved, &c., the position of the stomata is more undefined and 
apparently irregular. The number of the stomata is generally extraordinarily great 
in the epidermis of organs containing chlorophyll. In 54 species A. Weiss counted on 
one square mm. from i to 100 stomata, in 38 species 100-200, in 39 species 200-300, in 9 
species 400-500, and in 3 species 600-700 stomata. The origin of stomata is always 
the formation of a mother-cell, first of all by division of a young epidermal cell, which 
is sometimes preceded by several preparatory divisions in it ; this mother-cell becomes 
Fig. 85.— Development of the stomata in the ieaf of Scauin puTpurascens. A \&Yy yount^ stom.ita ; I-S one nearly 
mature ; e e epidermal cells ; the numbers indicate the successive order of the preparatory divisions. 
then more and more rounded off, and the Guard-cells of the stoma are produced from 
it by division. The variety of these processes up to the point when the cleft itself 
appears can hardly be explained in a few words; I prefer therefore to describe some 
examples more minutely. One of the simplest is afforded by the development of the 
stomata on the leaf of Hyacinthus orient alis, which has already been represented in 
vertical section in Figs. 61-64 (p. 77). The preparation for the formation of the stoma 
is here very simple. A nearly cubical piece of a long epidermal cell is separated by 
a septum, and this is the mother-cell of the stoma. It is divided by a longitudinal 
wall (/. e. by one parallel to the axis of growth of the leaf and at right angles to its 
surface) into two equal cells, which round themselves olf as they grow. How the 
splitting of the partition-wall takes place has already been depicted in Figs. 61-64, and 
can now easily be understood by the help of the surface-view in Figs. 84-86. In 
Equisetum Umosiim a similar appearance to that represented in Fig. 61 shows itself 
immediately after the first formation of the mother-cells of the stomata ; but the 
mother-cell undergoes in these cases three divisions, first one obliquely to the right, 
then one obliquely to the left, finally the middle cell is bisected by a wall at right 
angles to the surface. Four cells thus arise in one plane, of which the two outer ones 
grow more rapidly, while the inner ones are forced downwards and beneath them; 
