56 



OP WOOD IN GENERAL 



of the Forest of Dean {Q. Mobur, var. sessiliflora), are peculiarly 

 subject to this defect, which in the latter case has been doubtfully 

 ascribed either to the rocky character of the soil or to the swaying 

 to and fro of the tall trees by strong winds. This action of wind 

 bending the rings of wood alternately in opposite directions, in a 

 manner obviously calculated to tear them apart, may well explain 

 the occurrence of this form of shake in Poplars. Cup-shake has 

 also been attributed to frost, the rings of sapwood and heartwood 

 in a living tree containing varying proportions of water and the 

 outer layers being most hkely to freeze first. The explosive 

 rending of trees by frost, the noise of which disturbs the stillness of 

 night in the forests of North America, may in this way be sometimes 

 concentric in its action. This may explain the prevalence of this 

 defect in the swamp-loving Pitch Pine {Pinus austrdlis) of Vir- 



FiG. 37.— Cup- and heart-shake, 



ginia. Frost cannot, however, be the cause of the frequency of cup- 

 shake in the tropical Lignum- vitae ; but in this case the sun may have 

 produced an effect similar to that which sometimes occurs when 

 part of the cambium ring at the base of a stem is injured by a forest 

 fire. Lastly, in some Pines this defect is the result of the attacks of 

 certain fungi (Trametes), the " spawn " or " mycelium " of which 

 spreads as a felted mass of colourless mould especially in the cam- 

 bium. Cup-shake occurs most frequently at the base of the stem : 

 when of long standing, it is often accompanied by traces of rot, and 

 in many cases it is also associated with star-shake. 



Star-shake. — Star-shaJce consists in clefts radiating from the pith 

 along the planes of the pith-rays and widening outwards (Pig. 38). 



It occurs in many species and in trees of all ages. The clefts 

 may only extend a small distance and be so sHghtly open when the 



