ECONOMIC "WOODS OF THE UNITED STATES 



13 



mechanical support for the tree; (b) to afford means for the ascent 

 of sap from the roots to the foliage; (c) alternately to store up 

 and to give back products of assimilation, particularly starch. 



While the elements of secondary wood are subject to wide 

 variation, they may for convenience be referred to three principal 

 types, viz., (1) vascular, (2) fibrous, (3) parenchymatous. Between 

 these groups are transitional and specialized forms whose reference 

 to one or the other of these groups is often purely arbitrary. The 

 classification may be extended as follows: 



Vascular elements 

 True vessels 

 Tracheitis 



(wood) tracheids 

 ray tracheids 



Fibrous elements 



Wood fibres 



Septate wood fibres 

 Parenchymatous elements 



Wood parenchyma 



Ray parenchyma 



In the following table are shown side by side the important 

 differences in the distribution of the elements in typical secondary 

 wood of Gymnosperms and Dicotyledons. (See Appendix, p. 131.) 



Gymnosperms 

 True vessels absent. 

 Wood tracheids present and forming 



bulk of wood. 

 Ray tracheids present or absent. 

 Wood fibres absent. 

 Wood parenchyma present (except in 



Taxaceae), but usually subordinate. 

 Ray parenchyma present. 



Dicotyledons 

 True vessels present. 

 Tracheids present or absent; always 



subordinate. 

 Ray tracheids absent. 

 Wood fibres present. 

 Wood parenchyma present, and very 



often conspicuous. 

 Ray parenchyma present. 



From the above it is apparent that the wood of Dicotyledons 

 is more heterogeneous in its nature than that of Gymnosperms, 

 which is composed almost wholly of tracheids and ray parenchyma. 



References 



Solsredbh, H. : Anatomy of the Dicotyledons, Vol. II, pp. 1133-1168. 



DeBary, A.: Comparative Anatomy, pp. 458-500. 



Bohlger, G. S.: Wood, London, 1908, pp. 1-54. 



Stevens, W. C: Plant Anatomy, pp. 48-56; 72-112. 



Sachs, J. : Text-Book of Botany, pp. 92-102. 



Mell, CD.: A Confusion of Technical Terms in the Study of Wood Struc- 

 ture, For. Quarterly, Vol. IX, No. 4, 1911, pp. 574-576. 



: Classification of Woods by Structural Characters, Am. Forestry, 



Vol. XIV, April 1910, pp. 241-243. 



