EFFECTS OF LIGHT INTENSITY 731 



Plants grown in darkness frequently elongate more rapidly than those 

 given daily exposures to light, and if abundantly supplied with reserve 

 food, they may attain a greater total length. Foliar expansion, on the 

 other hand, is inhibited by darkness. 



Figdor (34) studied the influence of light on the structure of a South 

 African lily (Bowiea volubilis). This plant sends up a shoot from an 

 underground bulb. When grown in light, the shoot has a primary axis 

 and a number of side shoots. When grown in darkness, the primary 

 shoot develops about the same as in light, but the secondary shoots 

 remain short and undeveloped. Etiolated plants are generally very 

 succulent and hence much lower in percentage of dry matter than those 

 developed in light. The total dry weight produced by etiolated plants 

 is, of course, less than that of the seed or vegetative organ from which 

 they are grown, since some carbohydrate materials must be used up in 

 respiration, and not all of the dry material in the seed or primary organ 

 is available for the production of new growth. 



Etiolated plants grown for the same length of time and at the same 

 temperature as comparable green plants grown in the light will, of course, 

 be lower in total dry weight because they are deprived of the products 

 of photosynthesis. Whether light exercises a stimulative effect upon 

 the rate of utilization of stored food material in seeds or other organs is 

 still open to question. Lubimenko and Karisnev (63) studied the initial 

 rate of growth (dry-weight basis) of seedlings of certain cereals under 

 different intensities of daylight and in almost complete darkness. They 

 also followed the loss in weight of the residual seed. The plants in light 

 not only increased more rapidly in dry weight than those in darkness, 

 but they also tended to exhaust the seed more rapidly. They concluded 

 that light produces a direct action on the accumulation of dry matter 

 in plants during their purely "saprophytic" nutrition. Shirley (95) con- 

 ducted a similar experiment on maize seedlings, except that he had no 

 plants in complete darkness. He also found that seedlings in the light 

 exhausted the seed more rapidly than those in very weak light (too 

 low in intensity for appreciable chlorophyll formation); however, the 

 temperature was also higher in the light. When plants in light and in 

 darkness were grown at the same temperature, no difference could be 

 found in the rate of utilization of the stored products in the seed nor, 

 during the first 5 days, in the weight of the seedling. Thereafter, the 

 seedling in the light increased in dry weight more rapidly, but this 

 is attributed to the products of photosynthesis. It is unfortunate that 

 Lubimenko and Karisnev did not have accurate temperature control 

 and that Shirley did not make tests in the absence of carbon dioxide. 

 Further work on this problem should also be accompanied by chemical 

 analysis of the seed and seedling. 



