212 PLANT PHYSIOLOGY 



The proportion between the duration of light and darkness 

 is also of great significance. In the night, the leaves of plants 

 are transformed from producing into consuming organs; thus the 

 longer the day and the shorter the night, the more rapid is the 

 accumulation of dry weight. This probably explains why 

 beyond the Arctic Circle, where in summer the sun does not set, 

 plants accumulate considerable amounts of dry substance, and 

 with the application of proper methods of agriculture, meadow 

 grasses and truck crops may give in these regions very large 

 yields. However, the general rule of greater accumulation of 

 dry substance with the elongation of the period of daylight has 

 numerous exceptions owing to the great influence of this factor 

 upon the time of flowering and fruiting, and the appearance of 

 organs that respire intensely and thus consume large amounts of 

 assimilates. This question will be treated in greater detail in 

 Chap. XII, w^here the phenomenon of photoperiodism will be 

 discussed. 



Changes in temperature produce significant alterations in the 

 relation between photosjmthesis and respiration. This is the 

 reason why temperature also greatly influences the production of 

 plants. As a general rule, with increase in temperature, respira- 

 tion increases more rapidly than assimilation, and thus the bal- 

 ance between these two processes becomes less advantageous to 

 dry-weight accumulation. With a sufficiently intense and pro- 

 longed illumination, this influence of temperature is not so 

 effective; but when conditions are less favorable for photo- 

 synthesis, respiration may increase with temperature to such a 

 degree that the total plant weight will cease to augment and the 

 plants, instead of accumulating, will lose organic substance. 



This phenomenon may be observed, for instance, in green- 

 houses in winter; gardeners long ago recognized the necessity for 

 lowering the temperature in greenhouses during the darker 

 winter months. This has to be considered likewise in culti- 

 vating plants under electric light, which is always less intense 

 than sunlight. Here also, in most cases, it is profitable to grow 

 the plants at comparatively low temperatures. 



The enormous accumulation of plant mass in tropical climates 

 does not contradict these statements. Here the chief role 

 belongs to the duration of the vegetative period, which continues 



