356 Copeland . — The Mechanism of Stomata. 
Oenothera linearis , Mx. Type of Achillea. 
Passiflora lutea , L. Like Anemonella. 
Eucalyptus globulus. Cotyledon. Like Halophytes: much 
thickened, and no part of ventral wall thin : probably rigid. 
Character less extreme on leaf. 
Opuntia Rafinesquii, Engelm. Leaf. Many stomata rigid or 
abortive, but some motile. Amaryllis type, but with features 
of Achillea. 
Hydrocotyle Americana^ L. Type of Achillea : slender, with 
oblique dorsal walls. 
Osmorrhiza longistylis , DC. Type of Achillea. 
Nyssa sylvatica , Marsh. Like the Ericaceae, but thickening 
ceases less abruptly at ends. 
The Ericaceae. With the exception of Arctostaphylos 
Uva-Ursi , Spreng., of which I have had only herbarium 
material whose stomata are as figured by Westermaier ( 1 . c. 
Fig. 4) and are certainly very nearly rigid, the stomata of 
all the Ericaceae I have examined are mechanically very 
much alike. These subjects have been Azalia nudiflora , L., 
Rhododendron maximum , L., Gaultheria procumbens , L., Oxy- 
dendrum arboreum , DC (Figs. 48-51), Andromeda polifolia, , L., 
Pyrola secunda , L., and Chimaphila umbellata , Nutt. In 
sections these agree essentially with those of Vaccinium , 
which impressed Westermaier by their resemblance to the 
grass type. The thickening is so distributed as to leave the 
thin polar areas familiar in surface view in the Gramineae. 
But the ends are not much deeper than the middle, the 
dorsal walls are not at all straight and are usually oblique 
in cross-section, the inner wall is much thicker than the outer, 
very deep subsidiary cells are not often present, and in 
most Ericaceae, Oxydendrum arboreum and Gaultheria pro- 
cumbens being exceptions, more than one cell opposes the 
back side of each guard-cell. Older stomata of some of 
these plants are slightly motile ; others apparently not at 
all so. Movement which is undoubtedly like that of Achillea 
is very clear when they are younger and less thickened. 
The following measurements were made June 1, on a young 
