106 



BOTANY: PRINCIPLES AND PROBLEMS 



in this spring wood that most of the upward conduction of water 

 takes place. In the later-formed portion of the annual ring, the 

 water-carrying cells are fewer and narrower, and the bulk of the 

 tissue is composed of fibers. This summer wood is responsible 

 for most of the rigidity and strength of the stem. In large 

 branches and trunks, the older portion of the wood, consisting of 

 the first-formed annual rings at the center of the stem, in time 

 becomes dead throughout and ceases to perform its functions of 



Transverss 



Radial 



Tangential 



Fig. 60. — Transverse, tangential, and radial sections of wood. Both logs have 

 been cut transversely. In addition, the one at the left has been cut lengthwise 

 tangentially, and the one at the right, lengthwise radially. Each cut presents 

 the appearance characteristic of that particular plane of section. The -annual 

 rings and wood rays may be distinguished in all three sections. 



water-conduction and storage. It then constitutes the heart- 

 wood (Fig. 59) and is frequently distinguished from the outer 

 layers by its darker color. The living and functioning part of the 

 wood is its youngest portion and is known as the sap-wood (Fig. 

 59). This, of course, is on the outside of the woody cylinder, 

 and it is usually rather constant in width in any particular species, 

 its innermost ring being converted into heart-wood each year as 

 its outermost is added by the activity of the cambium. All 

 of the non-woody cells here (the parenchyma cells and ray cells) 

 are alive. 



Wood is usually cut along one of three distinct planes, and the 

 cut surface in each case presents a very different appearance 

 (Fig. 60). In describing a given wood it is therefore customary 

 to consider its characteristics as they are shown in these three 



