192 PLANT PHYSIOLOGY 



being dried in place. The universal joint attaching the clasp to a sup- 

 port permits it to be set in any desired position. An addition of small 

 mirrors set at 45 , thus permitting both sides of the leaf to be seen at 

 once, would make a desirable addition for demonstration, and even for 

 exact study. 



Stoma Quantities. The most extensive study of stoma numbers, 

 sizes, and distribution yet made is that by Weiss in "Jahrbticher fur wissen- 

 schaftliche Botanik," 4, 1865-66, 123, 196, and from his tables there are 

 selections in Detmer, 170, and especially in Goodale, 71. For our com- 

 mon greenhouse plants a comprehensive study has been made in my labora- 

 tory by Miss Sophia Eckerson, with results later to appear, probably in 

 the Botanical Gazette, Vol. 46. In synopsis they are as follows: 



Distribution. Only about two-fifths of our common greenhouse plants 

 have any stomata upon their upper surfaces, and then almost invariably 

 in far smaller number than upon the lower. Those having them upon both 

 surfaces are, in order of nearness to equality of the two surfaces, Wheat, 

 Oats, Indian Com, Windsor Bean, Horse Bean, Sunflower, Marguerite, Ivy 

 Geranium, Castor Eean. Only floating leaves of water-plants have stomata 

 upon the upper surface alone. The commonest plants having them upon the 

 lower surface only are Abutilon, Coleus, Poinsettia, Ficus, Fuchsia, English 

 Ivy, Heliotrope, Oxalis, Primula, Salvia, Senecio, Tradescantia, Tropaolum. 



Numbers. The most numerous stomata occur, in order of abundance, 

 on the under surfaces of Abutilon, Fiats repens, String Pean, Squash, Salvia 

 involucrata. The fewest occur in general upon those having the largest. 



Size. The largest stomata, in order of size, occur on the under surfaces 

 of Wheat, Tulip, Oats, Primula sinensis, Marguerite, and Wandering Jew. 

 There is in general an inverse proportion between number and size. 



Constants. Taking all the 37 plants of this study collectively, the num- 

 ber ranges from o through a mean of 121 to 484 per square millimeter, whence 

 we may derive a conventional constant of 0-100-500 mm 2 , or a mean 

 of over 100 millions to the square meter. The mean size of the open pore 

 is 17.7X6.7 (conventionally 18X6) microns, and the mean area is 92 (con- 

 ventionally 100) square microns y The total pore area for a square milli- 

 meter is therefore 11,132 (conventionally 10,000) square microns, which 

 means that when all the pores are open about one-ninetieth (conventionally 

 one-hundredth) of the epidermal surface is open. 



A very important question here arises as to whether transpira- 

 tion is regulated, directly or indirectly, by purely physical causes, 

 or whether the regulation is in part at least vital or physiolog- 

 ical. This subject has recently been studied by Livingston 

 through his new method of relative transpiration, viz., com- 

 parison of transpiration with evaporation from a standard 

 water-surface, and by Lloyd through the physiology of stomata. 



