ASSIMILATION OF CARBON BY PLANTS 



215 



Weekly Changes in Weight of Corn 



As may be seen from the table, the weight of the plants 

 increases continuously throughout the 15 weeks, except the 2 

 weeks after germination w^hen there is a considerable loss in dry 

 matter. The weekly increments of weight, expressed in grams, 

 increase rapidly during the first 6 weeks, but from the seventh 

 to the ninth week, they become more or less stationary. Very 

 significant are the figures showing the weekly increments in 

 percentage of the total weight of the plant. In the beginning, 

 this relation increases very rapidly, from a loss of 50 per cent 

 during the first 2 weeks to a maximum of 683 per cent in the 

 fourth week. After this maximum, the percentage decreases 

 gradually, though the absolute magnitude of the increase con- 

 tinues to augment. The cause of this gradual fall in the per- 

 centage of increase may be explained by the fact that during 

 the early stages of development the plant consists almost entirely 

 of leaves; and all the organic substance, formed during the process 

 of photosynthesis, is consumed for the production of new leaves, 

 which soon begin to participate in the process of photosynthesis. 

 Later, there begins a progressive development of stems and 

 reproductive organs, which participate little in photosynthesis 

 but consume a large amount of assimilates for their development 

 and in respiration. 



Plants of various types shoAv considerable differences in the 

 general course of accumulation of dry substance. Early-ripening 

 plants as a rule form stems and reproductive organs earlier, 

 causing a break in the process of accumulation, and ultimately 

 produce a lower yield than plants with more prolonged growth 

 period. Therefore it is advantageous to sow early varieties only 

 in regions w^here early frost or early drought compels the hastening 

 of the harvest. In regions with a long vegetative period, e.g., 

 in the southeastern United States or on the steppes of North 

 Caucasus with their warm and humid climate, it is preferable to 



