Leaf I. 



Leaf IL 



IOWA ACADEMY OF SCIENCES. 81 



really have but little exact knowledge of these curious little plant valves. The 

 number of stomata on a square inch of leaf surface is surprising. It varies all 

 the way from a few thousands up to hundreds of thousands. However, all com- 

 putations of stomata are only approximations. The number varies on different 

 portions of the same leaf, and the difference is often great. To get even an 

 approximation it is necessary to take sections from different portions of several 

 leaves and get an average. For an example of this variation take the Duchess- 

 apple. Counts were made on different parts of three leaves with the following: 

 results: 



f 29 stomata in field of microscope. 



I 26 



-, 20 



I -A " 



f 30 stomata in field of microscope. 



1 80 

 ] 27 " 



[ 28 



Leap III i ^^' stomata in field of microscope. 

 " ^ 38 " " 



A difference of one stoma in the field makes a difference of over 5000 on a 

 square inch. Thus it is seen that the number in the Oldenburg (Duchess of Olden- 

 burg; varies from about 120,000 to 200,000, while we get as an average 1-50 000- 

 per square inch. From the table below it will be seen that this is about the 

 average number in the varieties of apples examined by me. Prof. Bessey^ found 

 from 150.000 to 200.000 and Mr. Wellman' observed about the same number, while- 

 Lindley" gives but 24,000. 



To obtain accurate measurements of stomata is even more difficult than to obtain- 

 their number. They are so very small and it is so difficult to get them always- 

 under the same conditions. In this work, also, we must make a large number of 

 measuremt:-nts and take the average. Stomata on the same leaf vary considerably 

 in size and somewhat in shape. While the majority are elliptical in outline, some 

 circular ones will be found. In some species they are rectangular. To show how 

 stomata vary in size in leaves of the same tree we will again take the Oldenburg 

 apple. Stomata were measured on three leaves. 



The largest, the smallest and intermediate sizes were taken. L. stands for 

 length and W. for width: 



ilowa Hurt. Report, 18T9, p. 131. 

 5Iowa Hoit. Report. 187a. p. 117. 



6In his Introduction to Botany, p. 145, Lindley gives the number of stomata in thirty- 

 six species of plants, twenty-eight of which were computed by Thomson. 



