70 Studies in Forestry [CHAP. iv. 



those growing in shade. The latter are sometimes only about 

 one-third of the thickness of the former, and great differences in 

 size may even be noticed in the buds throughout the winter. If 

 young growth, that has hitherto stood in shade, be suddenly 

 exposed to the full action of light, these thinner leaves are not 

 always capable of carrying out the new and more energetic work 

 thrust upon them ; hence they often fall into a sickly condition, 

 which predisposes them to disease, and may even induce the 

 death of the plant. Thus, in areas regenerated naturally, the 

 clearance of the parent standards should be made gradually, 

 although the period over which it must be extended varies 

 considerably according to the kind of tree, and, for one and 

 the same species, according to the concrete conditions of the soil 

 and of the plants concerned. Broad-leaved deciduous trees can 

 in general accommodate themselves comparatively easily to rapid 

 transition from shade to light, owing to their leaves only lasting 

 for one season but in conifers this accommodative power is 

 necessarily more gradual, and is least in those genera that retain 

 their foliage for a long period, like Spruces and Silver Firs. The 

 poorer the soil of any locality, and more particularly the nearer 

 it approaches the minimum with respect to the water-supply 

 requisite for the normal transpiration of any species of woodland 

 tree, the greater is the necessity for a free supply of light in order 

 to carry out in as thorough a manner as possible the assimila- 

 tive process with the limited amount of sap and of nutriment 

 yielded by the soil. But the quantity of foliage decreases along 

 with these latter supplies. Hence it becomes of all the more 

 importance that the limited amount of nutriment should be 

 elaborated in a most thorough manner ; and this is only 

 possible with the comparatively free exposure of each individual 

 leaf to light and air. Or, in other words, this naturally leads to 

 the formation of a loose crown of foliage, instead of a dense 

 leafy canopy being maintained. 



Carbonic Acid or Carbon Dioxide (CO 2 ) becomes de- 

 composed under the action of light by the chlorophyll con- 



