4 o THE COTTON PLANT IN EGYPT CHAP. 



Cotyledon: Lower epidermis ... 

 Upper ,, 



Hypocotyl and stem 



Leaf : Lower epidermis 



Upper 



275 

 200 



20, &c. 

 176 to 116 



97 to 44 



The total number of these apertures is consequently 

 enormous. Such a seedling as that portrayed in Fig. 36, 

 already possesses about half a million. Nevertheless, the 

 cotton plant is not abnormally rich in stomata, though 

 the fact that they are found on both sides of the leaf 

 should be remembered. 



The functional capability of the immature stomata 

 found in very young leaves is not clear. The cuticle of 

 such leaves is yet thin, and in all probability it allows 

 some evaporation to take place. The density of the 

 stomata on such developing leaves is about the same as on 

 adults, so that fresh stomata are continually being differ- 

 entiated as the leaf expands. 



Further, with changes in the age of the leaf we find 

 changes taking place in the degree of those reactions 

 which the stomata exhibit towards environmental 

 changes. These are by no means fully understood, and 

 for the present we must confine our attention to the 

 normal leaves, fully grown, but as yet showing no signs 

 of senescence, which constitute the greater part of the 

 leaf area. The principal observations upon which this 

 account is based were made during May, June, and July. 



The study of changes in stomatal aperture under field 

 conditions has formerly been impracticable except by 

 personal observation, which always precludes continuity 

 in the records. The author succeeded, however, in 

 adapting Mr. Francis Darwin's Porometer method,* to a 

 self-recording system, and the first continuous automatic 

 records of stomatal aperture were thus obtained. 



In the Porometer, the leaf is made to seal an air-tight 



* Darwin and Pertz. 



