Ba) id 
The Construction of the Plant out of Cells. 61» 
nection between the intercellular spaces in the interior filled 
with air and the atmosphere. The interchange of gases. 
which is thus brought about 1s regulated by the guard-cells 
being sometimes more and sometimes less close together, so 
that the pore may be occasionally closed. 
Stomata are found in all the higher plants, from the 
_ Hepatic upwards. They usually occur in the green parts, 
more rarely in the coloured petals (as in the hyacinth, 
- crown imperial, &c.), [in the coloured bract or spathe of 
Richard@a|\, and in fruits and seeds (as the tulip). They are  — : 
wanting in those leaves which grow submerged in water ; and ’_ 
in floating leaves occur only on the upper surface. In most ~ 
Monocotyledons both surfaces of the leaf are provided with 
stomata, while in Dicotyledons they are generally almost or 
altogether absent from the upper surface. No general rule 
can be given for their number nor for their arrangement, 
which possesses a definite character only in a few groups of 
plants, as e.g. the Equisetaceze ; in Coniferze and Grasses the 
arrangement is invariably linear; in the Begonias they are — 
arranged in groups, &c. By far the most common size of 
the stomata ranges between o'0002 and o*0008 mm.; but - 
it may vary, as far as observations have yet been made, 
between 0-00459 (in Amaryllis formosissima), and o‘ooort 
(in Amaranthus caudatus). re 
Hair-like bodies or trichomes! are unquestionably epider- — 
mal structures, and may be found even on the youngest 
parts of the stem, leaves, and roots. Under this category 
are included hairs, bristles, stinging hairs, prickles, scales, - : 
glands, and glandular hairs. Whether the leaf-like ‘ paleze’ 
of ferns belong to this class, or should rather be regarded as 
leaves, is still doubtful. Aazrs arise by an elongation of 
particular epidermal cells (Figs. 87, 89); and if attached to 
a root are called voot-hairs. They are either simple or ~ 
branched, and consist of one or several cells. ABristles - 
1 See also the next chapier. 
