Chapter X — 195 — Loss and Retention 



ume of air space than sun leaves. Air space varies with water supply but 

 not uniformly among plants, and evergreens have a smaller volume than 

 deciduous plants. 



TuRRELL (1936) attempted to determine the area of internally exposed 

 cell walls and to compare this with total leaf area. A wide variation was 

 found in different species, and a comparison of leaves in high and low light 

 showed differences within a species. In general the palisade parenchyma 

 exposes much more surface per unit vokime than does spongy tissue. 

 Ratios of internally exposed tissue to external surface are given in Table 

 46. 



Table 46. — Ratios betiveen internally exposed cell walls and external leaf surface 



{data of TuRRELL, 1936) : — 



Ratio 

 of internal to 

 Plant Type external surface 



Bryophyllum calicynum succulent sun 7.9 



Syringa vulgaris mesomorphic shade 6.8 



Syringa vulgaris mesomorphic sun 13.2 



Ricinus communis mesomorphic shade 12.7 



Vitis viilpina mesomorphic sun 11.6 



Catalpa speciosa mesomorphic sun 19.2 



Gaultheria shallon xeromorphic shade 8.2 



Berberis nervosa xeromorphic shade 9.9 



Citrus grandis xeromorphic sun 17.2 



Eucalyptus globulus xeromorphic sun 31.3 



TuRRELL (1944) concluded that leaves having the largest internal ex- 

 posed surface should also have the highest transpiration rate, other condi- 

 tions being similar. To test this assumption he studied Nerium oleander 

 and Vinca rosea (periwinkle) grown under conditions of low and high 

 light intensity. These two genera with different stomatal distributions 

 were chosen in order to minimize the effects of stomatal control over trans- 

 piration. Stomata of periwinkle were considered 2.5 times too close to- 

 gether for maximum diffusion into still air; those of oleander 1.49 times 

 considering each stomatal pit as a unit and 5.7 times for the stomata within 

 a pit. The stomatal pits of oleander occur only on the dorsal surface, 

 whereas the stomata of periwinkle occupy both leaf surfaces. 



The xeromorphic leaves of both species, grown under high light in- 

 tensity, had greater internal : external surface ratios than the mesomorphic 

 ones developed in lower light. In spite of the fact that the internal transpir- 

 ing leaf surface averaged 8.1 times the external for periwinkle, and 15.6 

 times for oleander, the ratio of transpiration per unit internal leaf surface 

 to evaporation from a unit free water surface was only 0.012 for periwinkle 

 and 0.027 for oleander. These figures indicate that factors other than 

 internal evaporating surface are concerned in controlling the rate of trans- 

 piration from xeromorphic leaves. 



Stomatal Number and Distribution: — In spite of conflicting opinions {see Miller, 

 1938), the weight of evidence indicates that a genetic factor is involved in determining 

 the numbers and distribution of stomata in leaves. Any factor that limits the expansion 

 of a leaf increases the ratio of stomatal number to leaf area and also the pore space per 

 unit area (Salisbury, 1927). 



Maximov (1929a) gives a detailed consideration of Zalenski's (1904) investiga- 

 tions on the anatomical differences of leaves of the same plant. Zalenski, v^forking on 



