AGRICULTURAL, BOTANY. 719 



It was foimd that photosynthesis begins as soon as the minimum of light ab- 

 sorption has been attained, provided there is already present a minimum of 

 chlorophyll. The minimum of light required for photosynthesis varies, the 

 greater the amount of chlorophyll the less being the light requirement. The 

 energy of photosynthesis is said to increase with the amount of chlorophyll up 

 to a maximum, which is attained only in young leaves, older ones accumulating 

 an amount of chlorophyll in excess of their absorbing power in bright light. 

 Too strong light causes a diminution in the energy of photosynthesis, the de- 

 crease being more rapid as the chlorophyll content and the external tempera- 

 ture increase. The maximum of photosynthesis coincides with an optimum of 

 illumination, but the light optimum decreases with an increase in chlorophyll. 

 No direct relation was found between the temperature and chlorophyll content 

 as affecting photosynthesis. The specific energy of photosynthesis in various 

 species of plants is not directly influenced by the amount of chlorophyll present, 

 although too great a proportion of the pigment diminishes the activity of the 

 chlorophyll tissues of the plant. 



In a series of experiments to determine the production of dry matter and 

 chlorophyll in plants under different degrees of illumination, it was found that 

 the decomposition of carbon dioxid attained its maximum under red light and 

 its minimum under green light. On the other hand, the production of dry mat- 

 ter was at its minimum under the red light and its maximum under blue and 

 violet radiations. The production of dry matter under the red and yellow light 

 was less than the maximum energy of carbon dioxid decomposition under these 

 illuminations. This apparently indicates two stages in photosynthesis, the 

 first of which is characterized by the carbon dioxid decomposition and liberation 

 of oxygen, and the second in which there are photochemical reactions connected 

 with the transportation and utilization of elaborated material. It is pi'obable 

 that the chlorophyll grain assists in the second stage and that the more refran- 

 gible rays of light furnish the energy in this case. 



The production of dry matter under white light increases with the illumina- 

 tion and the decomposition of carbon dioxid up to a maximum, after which it 

 falls, although the carbon dioxid decomposition may still increase. This indi- 

 cates that the optimum of light for the production of dry matter in plants is 

 lower than that for the decomposition of carbon dioxid. An excess of light 

 absorbed by a leaf diminishes its production of dry matter. The unfavorable 

 influence of too great illumination is probably due to a retarding action of the 

 transportation and utilization of the organic materials elaborated by the chloro- 

 phyll tissues. 



The production of chlorophyll in a plant is regulated by the character of its 

 illumination. It increases with the light up to a maximum, after which it di- 

 minishes as the light increases. The maximum production of dry matter is 

 attained under an illumination that becomes more feeble as the chlorophyll con- 

 tent is increased. With a given plant the maximum dry weight is produced 

 under different degrees of light intensity, depending on the chlorophyll content 

 of the chloroleucites. A plant is able within certain limits to adapt itself to 

 light by increasing or reducing the amount of chlorophyll to correspond to the 

 strength of the light. This adaptation can not be effected except during the 

 development of the leaves. 



Two physiological types may be recognized among plants, those which pro- 

 duce little chlorophyll and have a high light requirement, and those which 

 form large quantities of chlorophyll and as a result are able to grow in com- 

 paratively feeble illumination. Plants require for the maximum production of 

 dry matter less light at a high temperature than at a lower one, light and tem- 

 perature within certain limits replacing one another. Photosynthesis is a 



