THE PLANT SKELETON 



249 



the accumulated annual increments of xylem which have fulfilled 

 their function of conduction and remain as the skeleton of the 

 tree. This wood is a homogeneous tissue composed almost en- 

 tirely of tracheids. A tracheid is an elongated cell, angular in 

 cross section, with walls which are slightly thickened and lignified 

 (fig. 171). The chief variation in the character of conifer wood is 

 the result of the difference between tracheids formed in spring 

 and those formed in summer. Spring tracheids have thin walls 

 and relatively large cavities, since they function chiefly for the 



TRACHEIDS 



SUMMER 

 TRACHEIDS 



SPRING 

 TRACHEIDS 



Fig, 172. — In a conifer the annual rings are due to the fact that the tra- 

 cheids of the spring wood have thinner walls and larger cavities than those of 

 the summer wood. 



conduction of water and are correlated with the increased flow of 

 ^sap which takes place at that time of the year. Tracheids formed 

 in summer are conspicuously diff'erent from those formed in early 

 spring, since they function more completely for support and less 

 I for conduction, hence have thicker walls and smaller cell cavities 

 (fig. 172). As a result the spring wood is more open and porous 

 :than the summer wood; the demarcation between the last 

 tracheids formed in summer and the first tracheids formed when 

 growth is resumed the following spring is the annual ring. Coni- 

 fers furnish man with a large proportion of his supplies of wood 

 and lumber, derived from this skeletal material composed en- 



