358 Copeland . — The Mechanism of Stomata. 
Mitchell a repens , L. Nearer Achillea than to Helleborus. 
Houstonia ciliolata , Torr. Achillea type. 
Houstonia caerulea , L. Achillea type. Ends, in surface 
view, suggest those of Juncus . 
Cephalanthus occidentalism L. Achillea type : slender. 
Campamda Americana , L. Helleboms type, but rather 
narrow. 
Lobelia puberula,M'K. Achillea type. Thickening of dorsal 
edge of inner wall runs to end ; rift rather long. 
Taraxacum officinale , Weber. Helleborus type. 
Senecio aureus , L. Achillea type. The subject was the 
radical leaf of a marsh variety. 
HeleniUm autumnale , L. Achillea type : thickening and 
length of rift as in Lobelia. 
Eupatorium ageratoides, L. f. Helleborus type. 
Eupatorium coelestinunim L. Achillea type. 
The number of stomata that I have had time to investigate 
would not warrant positive conclusions of any value in 
systematic botany ; but it is sufficient to show that no such 
conclusions could be expected from a wider range of study, 
except in such groups, like the genus Equisetum , as are 
sharply defined in other ways, morphological and ecological. 
The Glumiferae and Gymnospermae are such groups. The 
difference between the Achillea and Helleborus types, though 
a very great mechanical one, is insignificant morphologically, 
depending on the Relative depth and width of cells, which are 
very flat or very deep in neither case ; and it is not surprising 
that the two should often occur in the same genera, and that 
there should be every intermediate stage between them. The 
series of divisions leading up to the formation of the stoma is 
sometimes a character of taxonomic value, and the mechanism 
must to some extent be correlated with it. Thus the presence 
of anticlinal walls against the dorsal wall makes desirable 
some type better prepared to overcome local resistance than 
is the Amaryllis . 
Stomata of the Medeola type would not have to overcome 
this resistance ; but they must be large in order that the 
