42 PLANT PHYSIOLOGY 



is increased to 1%, there is a very rapid rise in photosynthetic 

 activity, according to Godlewsky. From this point on to 10%, 

 there is a slow rise, which requires increased temperature and illu- 

 mination to utilize, but 10% is the maximum, and if the amount 

 of carbon dioxide is increased beyond this figure there is a de- 

 crease in activity. An example of the many experiments carried on 

 in recent years on "fertilization" with carbon dioxide is the 

 work of Lundegardh, who burned alcohol lamps in a specially 

 prepared glass house, keeping the carbon dioxide in the air at 

 3-4 times the normal amount. Cucumbers grown under these 

 conditions yielded 30-45% more than the controls, and straw- 

 berries not only ripened earlier but had about 12% more fruit. 

 This experimentation can be done practically, of course, only in 

 greenhouses, where diffusion can be checked, although Riedel 

 near Essen, Germany, carried on some experiments in the field. 

 The effect of carbon dioxide on plant production is an extremely 

 important matter although, for the most part, it seems to be 

 almost entirely beyond human power to increase or diminish the 

 amount normally present. If carbon dioxide were more plentiful, 

 food would be even more abundant and cheaper. 



Daily Course of Photosynthesis. — With the present discussion 

 of limiting factors in mind, it is not difficult to picture the average 

 daily course of photosynthesis. Before daybreak, light is the 

 limiting factor, but some time before full daylight, the light has 

 ceased to be a limiting factor and photosynthesis is proceeding 

 at full speed, with carbon dioxide probably as the limiting factor. 

 Towards the middle of the day, a lack of water may become the 

 limiting factor, in which case the stomata may close somewhat and 

 the incoming carbon dioxide will be proportionately checked. 

 In the middle of the afternoon, photosynthesis generally slows up, 

 not because of external limiting factors but because of an internal 

 one, viz., the accumulation of starch in the leaves which hinders 

 more from being made. This drop in the photosynthetic curve is 

 later followed by a sharp fall (after dusk), owing to the lack of 

 light; and during the night, even in full moonlight, photosynthesis 

 is nil for all practical purposes. However, if the curve of photo- 

 synthesis were drawn to show the action during successive min- 

 utes, it would be extremely irregular. Maximov (1928) has shown 

 that photosynthesis may vary 25-100% in successive minutes 

 possibly because of the great variations in stomatal movement 



