PLANT PHYSIOLOGY AS RELATED TO PRUNIXG 



19 



is soon lost, or reduced Jjelow the needs 

 outer influences, sucli as sun and wind carr 

 by evaporation before it reaches the grow- 

 ing shoots. Where large branches are ringed 

 at their bases, however, so that at least 

 a part of the bark of the crown remains 

 connected with the roots, their growth 

 proceeds for many years with little or no 

 decrease of vigor or health and ceases 

 only when the decay of the barked portion 

 cuts off the supply of sap. 



The growth of the roots of trees 

 barked at the surface of the ground usu- 

 ally continues for one or two years after 

 barking, though whether this is due en- 

 tirely to the store of reserve materials in 

 the fieshy part of the cortical tissue, or 

 partly to root grafting from ncighlioring 

 trees, it is difficult to say. 



According to some authorities, the rise 

 of sap is confined tij one or twn-year-nld 

 rings in certain species, while in others as 

 many as five or six are concerned in the 

 business. In the latter the sap ascent 

 would probably be continued for several 

 years longer than in the former, ■ hile the 

 resistance offered by the wood to (!ecay 

 would also exercise its influence. In resi- 

 nous conifers the saturation of the Wdod 

 on the outside of the barked stem with 

 resin, which occurs when the cells and 

 cell-walls lose their water, acts as a pro- 

 tection against the cracking and drying of 

 the stem by atmospheric conditions, and 

 its infection by fungi. 



23. Felled trees develop sprouts. — .\n- 

 other interesting feature of the vitality of 

 woody tissue is the throwing out of ad- 

 ventitious shoots or "water-branches" by 

 the tree trunks felled in winter and left 

 lying with the bark on. Elm, oak, poplar, 

 willow, etc., on damp ground, continue 

 growing in this way for two or three 

 years, and the dwarf shoots on young 

 larch stems and branches left on the 

 ground also develop and keep green for a 

 month or two. Of course such growths 

 are dependent entirely upon the reser\e 



of the leaves, owing to 

 ying off most of the sap 





FIG. 14— CROSS SECTION 

 OF STEAl 



This microphotograph 

 shows how the stem of a 

 tulip tree (Liriodcndron tu- 

 lipifera) looks. Notice the 

 arcs of the annular rings, the 

 spoke- 1 ike medullary rays, 

 the bark (below) and the pith 

 (above). This stem is four 

 years old. 



