322 GROWTH OF PLANTS 



violet source show no fall in protective action for 24 hours after irradiation, 

 but there is some fall after 48 and 72 hours. The exposure of plants to 

 ultraviolet rays necessary to give appreciable amounts of vitamin D is 

 very much greater than the exposure of the animals themselves, sufficient 

 to give complete protection against rickets. 



It is evident that green plant materials grown in sunlight are a poor 

 source of vitamin D and that irradiation of green plants with artificial 

 ultraviolet sources is not economical not only because of the slow rate at 

 which this vitamin is formed, but also because of the later degeneration of 

 the vitamin during shipping and marketing. 



Ultraviolet irradiation of plants ^^ grown in soil under low light intensity 

 or during cloudy weather causes an increased absorption of ash, including 

 calcium or phosphorus, or both. Plants growTi in soil under high light inten- 

 sity and irradiated do not show a change in ash content. If plants are 

 gro\vn in midsummer when hght intensity is high, the light for growing has 

 to be cut down 65 per cent in order to make irradiation effective in increas- 

 ing ash absorption. Short irradiation (15 seconds) increases the ash in 

 the leaf and decreases that in the stem. Longer or repeated irradiation 

 increases the ash in both stems and leaves; this involves increases in cal- 

 cium or phosphorus, or both. Plants grown in sand show the response to 

 irradiation even when under high light intensity. The manganese and 

 magnesium content is not affected by irradiation. Plants grown in shade 

 are more easily injured by short ultraviolet than those grown in high light 

 intensity, but the effect on ash absorption occurred independently of injury. 

 The effective rays lie between 290 and 313 m/z, coinciding with the effective 

 antirachitic rays for the animal. Cabbage which is unable to form vitamin D 

 showed no increase in ash absorption. Application of irradiated ergosterol 

 to plants causes increased ash absorption. It is believed that ultraviolet 

 irradiation increases the mobilization and absorption of calcium and phos- 

 phorus indirectly by activation of the ergosterol in the plant tissue, that is, 

 by formation of vitamin D. 



Later work ^^ extends and clarifies these conclusions. Plants gro\vn in 

 absence of calcium and phosphorus show no increase in ash absorption due 

 to irradiation. The level of supply of calcium or phosphorus, and not 

 the ratio of calcium to phosphorus, determines the presence or absence of 

 response to irradiation. With a high ratio of calcium to phosphorus in the 

 nutrient medium, irradiation increases ash and calcium absorption; and 

 in a high ratio of phosphorus to calcium, it increases ash and phosphorus 

 absorption. 



Shorter ultraviolet rays of a mercury vapor arc in a quartz tube are very 

 injurious " to plants, but rays 290 m/x or longer are not injurious. The 

 injury increases rapidly as the ray length shortens from 290 to 200 m^i. 

 Radiations that give little or no injury with one application do not show 

 cumulative injury when the dosage is repeated daily. Filters that trans- 

 mitted rays as short as 286 mju injured tomato plants with 16.5 hours' 



