l86 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



According to Wiesner (1907), for Norway maple {Acer plata- 

 noides) the minimum light intensity under which photosynthesis 

 can still take place is i 55 of the total daylight, for birch 1/50, 

 and for beech somewhat less than for birch. The fact that in 

 light intensity of i 70 of the total daylight, such as prevails in 

 dense spruce forests, no green vegetation occurs under the shade 

 of the trees indicates that the minimum light intensity at which 

 a green leaf can still function must lie between 1/60 and 1/70 

 of the total daylight intensity. 



As far as woody species are concerned, Wiesner (1907) found 

 in the shade of European hornbeam {Carpinus betuliis), with a 

 light intensity of i,'58, an undergrowth of the following species 

 still in a fairly good condition : Beech {Fai^i/s sy/vafica), horn- 

 beam {Carpi /lus betulus)^ field maple {Acer campestre), common 

 elm {Ulmiis campestris) and dogwood {Cor mis scoiguinea). In 

 the same light intensity, how-ever, the undergrowth of elder {Sam- 

 bi/cus nigra), Eiwfiyiniis europceiis and E. verrucosus, was dying. 



The minimum light intensity at which green vegetation 

 disappears from under the shade of trees in the forest varies 

 considerably with the climate. Thus, in the Temperate Zone 

 no green vegetation occurs in the shade where the light is only 

 I 70 of the total daylight. In the Tropics the last vestige of 

 green vegetation disappears from under the tree cover when the 

 minimum light intensity falls to i 120 of the total daylight. 



The minimum intensity of light which is needed for the 

 formation of chlorophyll is much smaller than the minimum 

 required for photosynthesis. Thus, while the green leaves of 

 Acer platanoides cease to break up the carbon dioxide of the air 

 and form organic substance in a light whose intensity is i 55 of 

 the total daylight, the formation of chlorophyll still takes place 

 in a light which is i 400 of the total daylight; in some herbace- 

 ous plants, such as peppergrass or cress {Lepidium sativum), the 

 formation of chlorophyll still goes on at a light intensity of 

 1/2000, while in light whose intensity is i 50 of the total day- 

 light the leaves are just as green as in higher light intensities. 



The decomposition of carbon dioxide is produced chiefly by 

 the rays of the red portion of the spectrum, and the leaves in the 

 interior of the crown are able to assimilate, because the largest 

 part of the light which they receive penetrates not through the 

 green leaves, which absorb the red rays, but through the spaces 

 between the foliage. Chlorophyll may form in light which lacks 



