Meristems 



85 



smaller summer sieve tubes were formed. The various steps in the develop- 

 ment of xylem from cambium to heartwood, with particular reference to 

 changes in the cell wall, have been described by Bailey (1952). Lade- 

 f oged ( 1952 ) has published a detailed study of cambial activity and wood 

 formation in six conifers and 13 hardwoods in Denmark, based on obser- 

 vations from March to November. 



The character of cambial products, particularly on the xylem side, as 

 to the number and size of the cells and the thickness of their walls, is 

 influenced by various factors though the precise effect of these has had 



Fig. 4-23. Origin of ray initial in an angiosperm. Serial tangential sections of wood of 

 Trochodendron, showing origin of ray initial (stippled) from the end of a fusiform 

 initial. (From Barghoorn.) 



little experimental study. In temperate climates, during the cold weather 

 of much of the year cambial activity ceases. The contrast between the last 

 formed wood of one season and the first of the next makes it possible to 

 identify boundaries of the annual rings formed in each season. These are 

 absent in regions where growth is continuous. The relations between 

 climatic factors, particularly annual rainfall, and the width of these rings 

 has been studied ( Glock, 1955, and others ) . Lines of denser wood within 

 an annual ring may be related to rainfall differences in a single season 

 (Dobbs, 1953). Injuries from frost, fire, and insect attack can also be 

 recognized by their effects on the growth ring. These changes in the 



