The first xylem formed is called as protoxylem and later ones are called as metaxylem. There are four different patterns of arrangement of protoxylem and metaxylem. Of that four patterns, mesarch is one pattern. In mesarch xylem, protoxylem in primary xylem develops first in the centre of the strand and then continues to develop centrifugally and centripetally. Mesarch xylem is found in stem and leaves of ferns, pteridophytes. Thus, the correct answer is option D.