EFFECTS OF LIGHT INTENSITY 749 



in full sunlight. There appeared to be little difference in the number of 

 stomata per square millimeter, but there was a tendency for greater 

 concentration of stomata in the plants grown in the open. There were 

 more rows of palisade cells, and the cells were longer in the sun-plants. 

 The epidermis w^as thicker and there were more cells in the spongy- 

 parenchyma. The vascular bundles developed 26 per cent more cells in 

 the light, and the cell walls of the epidermis were much thicker in the 

 light. 



Penfound (72; 73) grew HeUanthus annuus, Polygonum hydro-piper, 

 and castor beans in full sunlight and under lath shades which transmitted 

 about 20 per cent light. Root length and thickness, stem thickness, and 

 leaf thickness were favored by full sunlight, while stem length was 

 greater in the shade. The depth of xylem in roots, hypocotyls, and stems 

 was greater in the light, and the cell walls of wood and bast fibers were 

 thicker. In full sunlight the leaf epidermal cells were larger, the palisade 

 and parenchyma layers deeper, and the number of stomata greater. The 

 conducting vessels per unit leaf area were also larger in full sunlight but 

 the rate of flow of water was the same. 



Steinbauer (101) cultivated for 10 weeks seedlings of Fraxinus 

 pennsylvanica in mineral nutrient solutions equivalent in osmotic con- 

 centration to atmospheric pressures of 0.01, 0.1, and 1.0, respectively, 

 and subjected to light intensities of 130, 70, 48, and 31 footcandles. 

 The plants which received only 31 foot-candles illumination succumbed 

 at the end of 2 weeks in all solution concentrations. Gain in dry weight 

 and in length of tops and roots was greater in the highest light intensity. 

 Best growth for these light intensities occurred in the solution with a 

 concentration equivalent to 0.1 atmosphere and poorest growth with 

 the equivalent of 0.01 atmosphere. 



Lubimenko (62), Burns (17), Grasovsky (43), and other investigators 

 have studied the light intensity required for a carbon dioxide balance — 

 that is, when the carbon dioxide given off in respiration will just be used 

 up in the photosynthetic process. They have found this to be attained 

 at rather low light intensities, about 1 to 5 per cent of full sunlight. 



The actual intensity required for a carbon dioxide balance depends 

 upon the previous treatment to which the plants have been subjected, 

 plants grown in strong light requiring a higher inten.sity for a balance. 

 Burns has considered this point at which the carbon dioxide balance 

 occurs as representing the minimum light requirements of the species in 

 question. Such an interpretation must, of course, take into consideration 

 plants growing under natural conditions, since they must build up in light 

 a reserve to carry them through the night. Furthermore, the respiration 

 rate increases, and probably also the point of balance, with increasing 

 temperature. This minimum requirement also assumes that light is 

 required only for the photosynthetic process. 



