STOMATA. 



73 



floating leaves they are confined to the upper surface of the 

 leaf. The leaves of certain plants, as those of monocotj-ledons 

 and those which take a vertical po- 

 sition, have tliem in nearly equal 

 numbers on the two sides ; but in 

 most cases the number on the under 

 exceeds that on the upper surface, 

 as will be seen from the table on 

 page 71. As regards the approxi- 

 mate number on leaves of average 

 size in some of our common plants, 

 the following figures may be of 

 interest : '■ — 



Nymphsea 7,650,000 



Bra.ssica oleracea, 11,540,000 



Helianthus annuus 13,000,000 



242. Water-pores. Directly over the extremities of the fibres 

 of the framework of many green leaves are found apertures in 



the epidermis which have no true guardian cells,* but which 

 closely resemble ordinary stomata in most other respects. •Owing 



^ That is, the bordering cells do not close under external influences. 



Fig. 54. Vertical section of stoma of Scdum apurium. (Strasburger. ) 

 Fig. 55. Water-pores in leaf of Bocbea cocciiiea. The left-baud figure shows both 

 an ordinary stoma (the lower one) and a water-pore (the upper), as seen on upper surface 

 of leaf Tbe rigbt-hand figure sbows tbe structure displayed by a vertical section. (Van 

 Tiegliim.) 



