158 Forestry Quarterly. 



"light sufficiency" — the intensity of light satisfying the needs of 

 the plant — is discussed for various plant types in their habitats 

 and also with reference to trees. The author makes a series, ac- 

 cording to light sufficiency, closely resembling the series from 

 tolerant to intolerant usually given in textbooks of silviculture. 

 The light intensity in forest shade and conditions under it are 

 discussed most interestingly, bringing scientific support for many 

 silvicultural practices. 



The dependence of light sufficiency on altitude and latitude is 

 discussed in the sixth chapter. Wiesner has shown, that between 

 the temperature of the medium (air, water) and the light suffi- 

 ciency there exists a relation in that a part of the light which 

 reaches the plant means a gain in temperature which is the greater 

 the lower the surrounding temperature. Hence, in warmer lati- 

 tudes a plant will show a lower minimum of light sufficiency, in 

 cooler, more northern latitudes a higher one; the needed light 

 supply increases towards the northern limit of the species, — a fact 

 which explains many phenomena of plant distribution and has a 

 bearing on silvicultural operations. 



The relation of light sufficiency and leaf fall, of light suffi- 

 ciency and mycorhiza formation — the mycotrophy increasing the 

 lower the minimum light sufficiency — and of its influence on the 

 color of leaves are dealt with. The foliage of summer-green 

 plants changes color as long as it grows, while in evergreens even 

 after they are full-grown the leaves darken for two or three years. 



Each species or variety exhibits a certain stationary tone of 

 green in the mature foliage, but the exterior and the innermost 

 foliage of the crown pale by and by, the first on account of excess, 

 the latter on account of deficiency of light, only the leaves under 

 medium light preserve their stationary tone of green. 



A physiological analysis of light sufficiency is attempted in a 

 suggestive chapter. Below the minimum of light sufficiency, the 

 formation of chlorophyll is retarded, and finally leaves die. As 

 Cieslar has experimentally shown (see pages 180-185 of this vol- 

 ume), with plants requiring high light intensity etiolation begins 

 even under quite intense light. Assimilation ceases as a rule at 

 the light sufficiency minimum. The light under which the birch 

 ceases to assimilate still produces good assimilation in beech. 



How the study of light relations by photometric methods can 



