CHAP, in.] Characteristics of Trees 51 



a greater height on the southern than on the northern sides, 

 in consequence of their requirements as to warmth. 



The root-systems of the various species of forest trees vary 

 greatly with regard to shape and to the depth to which they 

 reach ; but even shallow-rooting kinds derive advantage when 

 the soil over which they grow is deep, owing to the greater 

 supplies of nutriment within easy reach of their roots. Some 

 are characterized either by a strong tap-root, or else by a deep 

 heart-shaped root with stout ramifications, as in the Oak, Elm, 

 Pine, Silver Fir, Maple, Sycamore, Ash, and Larch ; others 

 have no pronounced tap-root, but develop strong side-roots 

 penetrating into the soil for a moderate depth, as in Beech, 

 Hornbeam, Aspen, and Birch; others again, like the Alder, 

 throw out strong side-roots, whence strands are sent down into 

 the soil ; whilst some are unmistakably shallow-rooting, as in 

 the case of the Spruce. Spruce, Aspen, and Birch require 

 least depth of soil, and Oak and Larch greatest. Of the other 

 trees, Scots Pine, Silver Fir, and Douglas Fir need deeper soil 

 than Austrian and Weymouth Pines ; whilst Beech, Hornbeam, 

 and Alder, although by no means shallow-rooted, can do with 

 a less depth than Elm, Maple, Sycamore, and Ash. 



In referring to von HonePs experiments regarding the tran- 

 spiratory power of the various species of trees per unit of 

 weight of foliage, it will be remarked (Chapter IV) that his 

 researches give us no reliable data for estimating the absolute 

 quantities of soil-moisture necessary for woodland crops so as 

 to supply their normal requirements throughout the annual 

 period of active vegetation. 



Practical experience, however, shows that Alder, Ash, Willow, 

 Poplars, Maple, Sycamore, and Elm, then Larch, Weymouth Pine and 

 Spruce, prefer soils with a considerable degree of moisture ; whilst 

 pedunculate Oak, Hornbeam, Birch, and Aspen, and in a less degree 

 Silver and Douglas Firs, also make greater demands than Beech and 

 sessile Oak, or Scots, Corsican, and Austrian Pines, which are the three 

 species that can best accommodate themselves to dry soils (the former on 

 those of a sandy, the latter on those of a limy nature). 



E 2 



