RESULTS OF EXPERIMENTS. 



11 



In view of the fact that Francis Darwin found the bloom in different 

 species of Trifolium to be most strongly developed on the leaf surface 

 having the greatest number of stomata. a count was made of the 

 stomata on equal areas of both surfaces of the leaves of wheat, oats, 

 and barley. In these cases the number of stomata as well as the 

 amount of bloom was found to be about the same on both surfaces 

 of the leaves. 



EFFECT OX THE THICKNESS OF THE CUTICLE. 



The following table shows the average thickness of the cuticle a 

 and outer epidermal cell wall (taken together) of the upper and 

 lower surface of the leaf of each of the different plant species grown 

 in the soils containing various concentrations of soluble salts and in 

 the control soils, the results of the measurements being expressed in 

 microns. 



Table I. — Thickness of the cuticle o?k/ outer epidermal cell wall {taken together of 



three species of plants grown in soils containing different concentrations of readily soluble 

 salts* 



Plant. 



Control 



(nonsaline 



soil) . 



Soil contain- Soil contain- 

 ing 1 per cent ing 1.5 per cent 

 total salts (es- total salts (es- 

 timated 0.7 timated 1 per 

 per cent so- cent sodium 

 diumchlorid i. chlorid). 



Soil contain- 

 ing 2 per cent 

 total salts (es- 

 timated 1.4 

 per cent so- 

 dium chlorid) . 



Wheat (Triticum durum) 



2. 6 2. 7 

 2. 4 2. 2 

 2. 5 2. 4 



3. 2 3. 

 3. 3. 

 3. 3. 



3.2 

 3.2 

 3.2 



3.2 

 3.3 

 3.3 



3. 3 3. 3 





3. 2 3. 2 



Barley (Hordeum distichum 







* The figures represent in each case averages of about 100 measurements. 



It will be seen from the above table that the thickness of the cuti- 

 cle increases with the concentration of salt in the soil. In every 

 case the thickness of the cuticle on both surfaces of the leaves is 

 greater in plants grown in the soil containing an excess of soluble 

 salts than hi the control plants, and increases with the concentration 

 of the total soluble salts present. The single exception to the lat- 

 ter rule was the lower leaf surface of the leaf of oats (Arena sativa) 

 grown in soil containing 1.5 per cent of total salts, the average 

 thickness of the cuticle having been in this case slightly greater 

 than in the soil containing 2 per cent of total salts; but the differ- 

 ence is unimportant and is within the limits of experimental error. 



In wheat, oats, and barley, so far as these experiments show, 

 there seems to be little difference in the thickness of the cuticle 

 between the upper and lower surfaces of the leaves, whether in soils 

 containing an excess of readily soluble salts or in the nonsaline soils. 



"All measurements of the thickness of the cuticle were made exclusive of the 

 waxy deposit, which had been previously dissolved off by the addition of xylol. 



134 



