CELLULAR STRUCTURE OF LEAVES 249 
NUMBER AND DISTRIBUTION OF STOMATA PER SQUARE 
MILLIMETER OF LEAF SURFACE 
Plant Lower Surfaee | Upper Surface 
Lilac (Syringa vulgaris)........................ 330 0 
Alfalfa (Medicago sativa)................. ae 160 160 
Bean (Phaseolus vulgaris)............... 281 40 
Tomato (Lycopersicum esculentum) 130 12 
Chertysccus o: causieoy Spices 62 gadis : 160 0 
Pumpkin (Cucurbita pepo)..................... 269 28 
Oats (Avena sativa). .........00. 00.00 0...000 0 { a | Ee 
Corn (Zea Mays)... . 2.0.0.6 0 0c c cece ees { ee j 
Although stomata are most numerous on leaves, they occur in 
Flowering Plants wherever there is green tissue to be supplied 
with gases. They are common on fruits, green twigs of trees, 
and are present on nearly all parts of the aérial stems of herba- 
ceous plants. On the older twigs and trunks of trees, the stomata 
are represented by the lenticels which are the structures into 
which stomata are transformed as the stem becomes enclosed in 
bark. The stomata are distorted and transformed into lenticels 
partly by the stretching of the bark and partly by the tissue 
which grows up from beneath and crowds into the stomatal 
openings. 
In order to get a view of the epidermis in cross section and to 
study the chlorenchyma and veins of a leaf, a thin section must 
be made and highly magnified as shown in Figure 232. In this 
view an ordinary epidermal cell is rectangular, has a large central 
cavity separated from the cell walls by only a thin layer of proto- 
plasm, and has the outer wall more thickened than those within. 
The continuity of the epidermis is interrupted by the stomata, 
each of which opens into an air chamber in the mesophyll just 
beneath. 
The chlorenchyma is composed of thin-walled cells, having 
thin layers of protoplasm in which the characteristic green bodies 
(chloroplasts) are located. In most horizontal leaves, the cells 
of the chlorenchyma are differentiated into two distinct groups, 
the palisade and the spongy tissue. The palisade tissue is next to 
the upper epidermis and consists of one or more rows of compact 
elongated cells in which chloroplasts are especially abundant. 
