108 ORGANIC STRUCTURE. 



nourished, or from there being more branches on one 

 side than on others. Sometimes the south, in other 

 instances the north side is most prominent. The 

 under side of leaning trees, and of all branches, are 

 extended further from the centre than the upper 

 side*. A wound on one side will determine the 

 greatest share of the growth to the opposite. 



Fig. 34. 



Example of the manner in which a wound received in the fifth 

 year is healed. A transverse section. 



Neither are the annual layers equal in size ; some 

 are thicker than others ; caused, no doubt, by uncom- 

 monly favourable seasons ; a tree makes more pro- 

 gress in a moist and warm summer than in one that 

 is cold and dry. There is also difference in the thick - 



* This is attributed by some physiologists to the effect of gravi- 

 tation on the fluids ; others imagine that the upper side being more 

 exposed to ai and light, the sap is thereby exhaled away more 

 rapidly, and consequently the tubular structure is less distended. 



