36 TREES IN" WINTER 



is the finished product. Sunlight is the energy which runs the 

 machinery. 



The fact that sunlight is necessary for the formation of plant 

 food in the leaves, explains the poor development of branches that 

 are partially shaded. The Post Oak, shown on page 101, w^as in- 

 terfered with by a neighboring tree on the right. The scant growth 

 of limbs on this side is the result. The lower branches of trees 

 in the woods die and are ^'^self-pruned" when their leaves are so 

 shaded that they are no longer able to make sufficient food for a con- 

 tinuation of growth. It is for this reason, that all forest-grown 

 trees tend to produce tall trunks with but little branching (fig. 19) 

 while trees in the open (fig. 18) branch more freely and are there- 

 fore better able to express the habit characters peculiar to the 

 species. 



Species of trees differ in their ability to tolerate shading. There 

 are "tolerant" forms like the Hemlock, the Beech and the Sugar 

 Maple that are able to grow under the shade of other trees, and 

 "intolerant" forms like the Poplars and Birches that are killed by 

 a relatively small amount of shading. In planting groups the rela- 

 tive tolerance of the forms represented should be taken into con- 

 sideration. 



Although the leaves are arranged in a position favorable to illum- 

 ination, they are at the same time exposed to conditions favorable 

 to evaporation. It has been estimated that a Beech 110 years old 

 "transpired" through its leaves approximately 2,250 gallons of 

 water during a single summer and that an Oak with some 700,000 

 leaves transpired about 180 gallons daily. Such a considerable 

 amount of water lost by the leaves must be made good by the roots 

 or the tree will perish from thirst. A diminished root absorption, 

 brought about by injury during transplanting or caused in any 

 other way, can be neutralized by pruning the crown and thus reduc- 

 ing the amount of leaf surface exposed to evaporation. 

 ' Trees that drop their leaves all at once are called "diciduous" in 

 distinction to our Evergreens which retain their foliage through- 

 out the winter. The latter, however, are not stnctly evergreen. 

 In the White Pine for example (page 211), leaf scare can be found 

 in increasing numbers on second and third year's growth and show 

 that in this species the fall of the foliage is gradual and that its in- 

 dividual leaves usually do not remain on the tree more than three 

 years. The evergreen character is generally associated with cone- 

 bearing forms. The Holly is the only broad-leaved tree of the 



