EXTERNAL CAUSES OF GROWTH AND FORMATION. I 307 



versely, and BERTHOLD (1882) has demonstrated superelongation in Algae 

 placed in somewhat too bright light. Diminution, however, may clearly be 

 seen to follow further increase in light intensity, finally resulting in a cessa- 

 tion of growth. It is not difficult to prove an increase in leaf -surf ace as a con- 

 comitant of an increase in illumination if we compare etiolated with normal 

 leaves. STAHL (1883) has, however, calculated that in moderate light in shady 

 situations leaves become larger than in direct sunlight, and has shown that 

 beech-leaves, exposed to sunlight are only half, and leaves of the elder only 

 a quarter, the size of those grown in the shade. Thickness of the leaf stands 

 in close relation to area ; the thickness increases with the reduction of surface 

 and vice versa. It is known also that the anatomical structure of the leaf is 

 greatly affected by light. Elongated palisade tissue is 

 the characteristic feature of leaves exposed to sunlight, 

 spongy mesophyll of leaves which are shaded. While 

 many plants demonstrate themselves to be light or 

 shade plants by their leaf anatomy there are other im- 

 portant adaptations which are worthy of note (Fig. 90). 



Experimentally it may be shown that each bud can 

 be made to unfold, and in doing so, if the light be suf- 

 ficient, it becomes a normal shoot, if insufficient an 

 etiolated one. In nature weakly illuminated buds do 

 not produce etiolated shoots, they simply do not develop 

 at all. Evolution of the bud takes place only if the 

 light be sufficiently intense, and the degree of intensity 

 varies greatly for different plants. WIESNER (1893-1900, 

 summary, 1902) has provided us with accurate measure- 

 ments on the subject which lead us to numerous impor- 

 tant results. A few of these only can be quoted on the 

 present occasion. [Compare WIESNER, 1904 and 1905 ; 

 CIESLAR, 1904 ; HESSELMAN, 1904.] WIESNER used the 

 BuNSEN-RoscoE method for measuring light intensity, 

 a method well adapted for estimating the highly re- 

 frangible rays only, i. e. those which are of most impor- 

 tance to the plant (p. 311). WIESNER has determined the 

 degree of light intensity under which a number of plants 

 will live in different surroundings. First he determined, 

 by the BUNSEN-ROSCOE method, the ' absolute photic ration ', and then 

 deduced therefrom the ' relative photic ration ' falling on the plant. If the 

 plant is able to live, on the other hand, in full sunlight, and also in a light 

 intensity reduced to one-tenth of the maximum, WIESNER says the relative 

 photic ration lies between one and one- tenth. 



WIESNER gives the following data for Vienna : 



Fig. 90. Transverse sections 

 through the leaves of the cop- 

 per beech. /, illuminated ; //, 

 shaded. After NoRDHAUSEN 

 ( I9 03, PI. 4). 



Buxus sempervirens 

 Beech (enclosed) 



(open ground) 

 Quercus pedunculate* 

 Betula verrucosa 

 Larix decidua 



Relative photic 

 ration. 



to ita 

 to -h 

 to ^ 

 to & 

 to i 

 to 



Minimum of absolute photic ration 

 calculated by the BUNSEN-ROSCOE 

 method. 



012 



.015 



.021 



.050 



.144 



For one and the same species the higher the latitude or the altitude the 

 more the light that is required. Its absolute and relative photic ration in- 

 creases with the diminution of temperature. Thus the minimum of the relative 

 photic ration in the case of Acer platanoides alters from one-fifty-fifth near 



X 2 



