TREE ROOTS : THEIR ACTION AND DEVELOPMENT. 7 



examined and will be used for the purposes of description : — 

 'Picea sttchensis, Picea excelsa, Pinus montana, Pinus sylvestris^ 

 Abies grandis, Tsuga albertiana, Pseudotsuga Douglasii, Larix 

 europaea, Thuja gigantea and Sciadopitys verticillata. 



I. Picea Sitchensis (Sitka or Menzies Spruce). 



In the Sitka spruce, as in the Norway spruce, there are 

 no nodules or coral-like growths formed on the roots, and a 

 macroscopic inspection of the roots for the presence or absence 

 of the fungus forming the condition of mycorhiza is, therefore, 

 not always reliable. At most when the fungus is present only 

 a slight thickening or shortening of the rootlet occurs, although 

 generally speaking there are more rootlets produced, and there 

 is a tendency towards clumping ; but this is nothing more than 

 a tendency and, therefore, only appreciable when a plant without 

 mycorhiza is present for the purpose of contrast. 



The fungus is found mainly on the fine absorbing rootlets, 

 but may not be confined to them solely, sometimes extending or 

 persisting on the older parts of the root-system. New rootlets 

 carry the fungus web with them as they arise, so that the new 

 rootlet has its mantle of fungal tissue from the very beginning 

 in many cases (Fig. i). 



A typical section of a fungus-bearing rootlet shows to the 

 outside of the cortex a layer of mycelium with strands passing 

 inwards between the cells of the cortex, and occasionally strands 

 penetrating the cortical cell walls (Fig. 2). Variations occur 

 even on the root-system of the same plant, or more markedly 

 between plants from different soils as regards : — 



1. The degree or depth of penetration of the mycelium 



between the cortical cells ; 



2. The penetration of the cortical cell walls ; 



3. The density of the enveloping web ; 



4. The type of mycelium forming the enveloping web. 

 The furthest depth of penetration of the intercellular mycelium 



has been found to be to within a distance of one cell layer from 

 the endoderm. On the other hand, no intercellular mycelium 

 may be present, and this is typical of the diffuse type of mat or 

 web to be mentioned later. In this case the fungus is present 

 solely on the outside of the rootlet. Instances occur where 

 filaments from the enveloping web penetrate the outer cell walls 

 of the cortex, and proceed directly towards the endoderm. In 



