ECONOMIC WOODS OF THE UNITED STATES 11 



PRIMARY WOOD 



At the growing apex of a stem is an undifferentiated tissue 

 composed of very thin-walled cells essentially all alike. This tissue 

 is known as the primordial meristem. 



Below the apex the primordial meristem becomes differentiated 

 into three distinct parts, viz., (1) the protoderm at the outside, 

 (2) the procambium strands, and (3) the fundamental or ground 

 meristem. These three regions or tissues are themselves subject to 

 further differentiation and are called primary meristems. The 

 protoderm changes into the epidermis; the ground meristem into 

 pith, primary rays, pericycle, and primary cortex; the pro- 

 cambium strands into vascular bundles, which are disposed in a 

 circle around the pith and separated from each other by the 

 primary rays. The vascular bundles are composed of three parts, 

 an inner called the xylem, an outer called the phloem, and, separ- 

 ating the two, a thin layer of generative tissue, the cambium. 

 These tissues, being the direct development of the cells of 

 the procambium, are termed primary (primary wood proto- 

 xylem and metaxylem and primary phloem), in contradistinction 

 to the tissues generated by the cambium, which are termed 

 secondary. 



Primary wood is relatively unimportant, though of scientific 

 interest because of its peculiar structure, which in many ways 

 differs from the other wood of the stem. Thus in Angiosperms, 

 wood fibres are usually wanting and tracheids are not common 

 in the primary wood, while in the secondary wood fibres are 

 always present and tracheids commonly so. In Gymnosperms the 

 vascular elements of the primary wood are indeterminate in 

 length, marked with spirals and for the most part devoid of pits 

 in their walls, while the corresponding elements in the secondary 

 wood are of determinate length, rarely marked with spirals and 

 always pitted. 



References 



STEVENS, WILLIAM C.: Plant Anatomy, pp. 25-45. 

 PENHALLOW, D. P. : North American Gymnosperms, pp. 38, 40. 

 DEBARY, A.: Comparative Anatomy, p. 321. 

 SACHS, J.: Text-Book of Botany, p. 574. 



