222 BOTANICAL GAZETTE [SEPTEMBER 
shoots of the same plant), this precaution being observed in order to mini- 
mize any possible abnormalities of single plants. 
In order to measure the size of the stomata, which involves the length 
and breadth of the guard cell apparatus when closed, and the length and 
breadth of the pore when most widely open, the same general method was 
used, except that, in order to prevent the closure of the pore through 
any possible wilting of the leaf, the epidermis was removed while the leaves 
were still attached to the plants. The pores of the stomata were found as 
a tule to be widest open at about 10 aA. M., and accordingly the material 
was taken at that time. The measurements were made with an ocular 
micrometer, carefully valued by comparison with a stage micrometer. As 
in the case of the countings, thirty measurements were made upon mate- 
rial practically taken at random from three plants, and the figures in the 
table represent the mean of these. Only the linear dimensions of the pore 
are given in the table, but the area can very easily be determined by treating 
the opening as an ellipse,+ though it is to be remembered, as BROWN and 
Escoms have shown,5 that in the passage of gases through stomata, it 1s 
the linear dimensions, and not the area, which is important. 
The data of the accompanying table may be summarized as follows. 
The stomata of our common greenhouse plants occur chiefly upon the under 
surfaces of the leaves, only about two-fifths (43 per cent.) having any upo? 
the upper surface, and those almost invariably far less numerous than those 
upon the under. The most numerous stomata occur (in order of abun- 
dance) in Abutilon, Ficus repens, Phaseolus vulgaris, Cucurbita Pepo, ant 
Salvia involucrata. The largest occur (in order of size) in Triticum sali- 
vum, Tulipa, Avena sativa, Primula sinensis, Chrysanthemum jrutescens, 
and T’radescantia zebrina. Ina general way, there is an inverse proportion 
between number and size of stomata. Taking all of the plants collectively, 
the number of stomata ranges from o through a mean of 121 to 484 per 
square millimeter, or, in general terms, they average over 100 to the square 
millimeter. The mean size of the open pores is 17.7X6.7 Bs and peer ; 
“ ; 
area is 92 square ». The total pore area for a square millimeter O° ™ 
therefore, is 11,132 square » (12192), which means that when the pores 
are open, one-ninetieth (or in round numbers over one-hundredth) of the 
epidermal surface is open. : 
Some other points worthy of mention in connection with the practical 
study of stomata in the laboratory are the following: 
HengtO 5, Preadith 
2 
4 Area of an ellipse = xr. 
5 Nature 62: 212. 
