PHOTOPEFTODISM 703 



or without flowering and fruiting, in plants of both groups, when they 

 are exposed to a short day. These authors suggest that the striking 

 effects of the hght period are the result of a twofold action of light, first, 

 its indirect action through the medium of carbohydrate produced in 

 photosynthesis and, second, its direct activating effect on chemical 

 reactions taking place in the embryonic tissue of the growing point. 



That the contrast in response of long-day and short-day plants to 

 length of day cannot be explained on the basis of photosynthesis alone is 

 indicated by the fact that artificial illumination of exceedingly low 

 intensity when used to lengthen the daily light period is capable of repro- 

 ducing the characteristic formative effects of high-intensity long-day 

 illumination. Moreover, Tageeva (70), in Maximov's laboratory, has 

 made direct comparison of the intensity and the daily course of assimila- 

 tion, as measured by absorption of CO2 and increase in dry weight, in the 

 oat (a long-day type) and millet (a short-day type) under natural growing 

 conditions and exposed to a long day and a short day, respectively. 

 There was no significant difference in the daily assimilation curves in the 

 long-day and the short-day types of plant nor in assimilation intensity 

 in the two types. It appears, therefore, that there is no definite relation 

 between assimilation in the long-day and short-day plants and their 

 photoperiodic responses. It was found, also, there is no fixed relation 

 between assimilation intensity and the accumulation of dry matter in 

 different day lengths. The sharp contrast in accumulation of dry 

 matter between long-day and short-day types exposed to a long or a 

 short day is due not so much to difference in working capacity of the 

 chlorophyll apparatus as to difference in the utilization of the assimilate. 



Considerable experimental work has been done to determine the 

 applicability of the C/N ratio of Kraus and Kraybill. Auchter and 

 Harley (7) found from preliminary studies that the short-day plant, 

 Biloxi soybeans, which quickly flowered in a short day, contained at 

 flowering time a relatively high percentage of total sugars and starch 

 and a low percentage of total nitrogen. The plants exposed to the full 

 summer day showed similar relations when they later reached the flower- 

 ing stage in response to the decreasing length of day. Plants darkened 

 at midday were much delayed in flowering and did not fruit, and in these 

 plants the percentages of sugar and starch were low, the soluble nitrogen 

 was low, but the insoluble nitrogen was relatively high. However, 

 plants exposed to continuous light did not flower but, nevertheless, when 

 analyzed were found to contain proportions of starch and total carbo- 

 hydrates to nitrogen quite similar to those in the short-day plants. In 

 experiments previously referred to, Gilbert (30) obtained in Xanthium 

 pennsylvanicum, exposed to conditions of high temperature and short day 

 or low temperature and long day, ascending C/N ratios as the plants 

 approached flower-primordia formation. 



