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PLANT MORPHOLOGY 



The primary wood that forms after the protoxylem is differentiated is 

 metaxylem. In dicotyledons the number of protoxylem strands is com- 

 monly 4 or 5, while in monocotyledons it is generally more, often 15 or 20. 

 Phloem occurs as separate strands lying between the groups of protoxylem 



Fig. 311. Cross section of the central portion of a root of baneberry (Actaea alba), 

 showing primary tissues and beginning of formation of secondary tissues by the cambium, 

 which lies between the secondary xylem and phloem, X200; end, endodermis; per, peri- 

 cycle; pp, primary phloem; sp, secondary phloem, px, protoxylem; mx, metaxylem; sx, 

 secondary xylem. 



elements. Branch roots arise in the pericycle directly opposite the proto- 

 xylem strands. They then grow outward through the cortex (Fig. 312). 

 Except in fibrous roots, a cambium arises between the primary xylem 

 and phloem and cuts off secondary vascular tissues — secondary xylem on 

 the inside and secondary phloem on the outside (Fig. 311). Soon a more 

 or less continuous cylinder of secondary vascular tissues is formed. 



