RELATION OF LIGHT-INTENSITY TO ADVANCE GROWTH. I 53 



ration (being the measure of the Hght-intensity) is plotted 

 horizontally. 



Taken as a whole, the curve for oak (Fig. i) indicates that 

 as the light-intensity increases the number of seedlings per unit 

 of area of the advance growth increases. The first part of the 

 curve shows the two to be almost directly proportional. In the 

 second part of the curve, it is observed that large variations in 

 the light-intensity are associated with comparatively small 

 changes in the number of seedlings. In this region of the curve, 

 however, it is recognised that the disturbing influences of weed- 

 growth and exposure enter in, and this may account to some 

 extent for the flattening of the curve. Thus, as evidenced by 

 the form of the graph, the number of seedlings seems to be 

 tending towards a maximum at point A, where the light-intensity 

 is one quarter the intensity of full daylight. 



The curve for beech (Fig. 2) is of a different nature. In the 

 first place, there are indications that, in the neighbourhood of 

 point F, the number of seedlings approaches a maximum ; in 

 other words, the light-intensity at this point (one-fortieth the 

 intensity of full daylight) approximates to the optimum. The two 

 parts of the curve appear fairly symmetrical about this optimum 

 point. In addition, each part of the curve shows that for a small 

 variation in the light-intensity, there is a comparatively large 

 variation in the number of seedlings. 



Now any comparison between the two species based on their 

 respective graphs must necessarily be of a very general nature, 

 because of the comparatively wide error limits of the experiments. 

 Such a comparison, however, indicates that in the case of beech, 

 the light-intensity corresponding to the maximum number of 

 seedlings is approximately one-tenth of the light-intensity at a 

 similar point in the case of oak. Again, the maximum number 

 of seedlings per "8 square pole in the case of oak is about half 

 the maximum obtained for beech. Finally, it would seem that, 

 under successful natural regeneration, in first youth as in mature 

 age, the number of beech trees on a definite area considerably 

 exceeds the number of oak trees. 



The question of the light-intensity at which different species 

 thrive best appears to require further elucidation. It is 

 stated that, for the majority of forest trees, the optimum light- 

 intensity approximates more nearly to the maximum amount of 

 light available for the use of the tree than to the minimum amount 



VOL. XXVI. PART II. L 



