X] POSITIONS AND NUMBERS OF STOMATA 99 



and stems, and from the covering of the roots. They 

 occur in very variable numbers on different plants, and 

 on different organs of the same plant : they are usually 

 most abundant on the true foliage-leaves, less so on the 

 sepals and axial organs, but are commonly found in some 

 degree on all the floral parts, and even on the integuments 

 of the ovule. 



The presence of stomata, and their communication 

 with the intercellular spaces of the plant, can be easily 

 demonstrated by the following simple experiment. A 

 leaf of Ranunculus Ficaria, or of an Onion, &o, is fixed 

 into a glass tube by means of gelatine poured around the 

 petiole or other part in the tube. On immersing the 

 lamina in water it is easy to force air-bubbles through 

 the stomata by blowing, or to inject the intercellular 

 spaces by sucking. 



The positions and the numbers of stomata per square 

 mm. of leaf-surface vary with the species of plant. Gener- 

 ally speaking they are most numerous on the under 

 surface of ordinary thin foliage-leaves, and fewest on the 

 thick leaves of succulent plants : facts in accordance with 

 the transpiration and other requirements of such plants. 

 Similar correlations between function and position are 

 found in the restriction of the stomata to the upper sur- 

 face of floating leaves (Potamogeton natans, Hydrocharis, 

 Water Lilies, &c.) ; whereas they are usually confined to 

 the lower surface in ordinary aerial leaves whose blades 

 are horizontal, or at least are far more numerous there 

 e.g. Hornbeam, Birch, Pear, &c. and especially in leathery 

 polished leaves e.g. Holly they are confined to the 

 lower surface: again in upright leaves (Iridew, Amaryl- 

 lidece) and phyllodes, and leaves which hang with their 

 edges up and down (Australian Mimosew, &c.) they are 

 usually equally abundant on both sides. 



72 



