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PLANT MORPHOLOGY. 
Secondary Structure. — While monocotyledonous roots 
grow in length, they do not grow perceptibly in thick- 
ness, so that, for instance, a sarsaparilla root, which may 
be nearly three meters in length, will show but little 
variation in thickness throughout its entire length. 
Dicotyledonous roots, however, soon begin to grow in 
thickness, as well as in length, the latter being less 
marked than in the monocotyledons. 
In monocotyledons the primary structure of the root 
is for the most part retained, the only change being an 
increase in thickness of the walls of some of the cells. 
On the other hand, the increase in diameter of dicoty- 
ledonous roots is accompanied by marked changes in 
the primary structure. Some of the cells at the 
periphery of the ducts and upon the inside of the sieve 
become meristematic, forming a continuous zone or 
ring, known as the cambium. On the outside of the 
cambium not only sieve cells but bast fibers and bast 
parenchyma may be developed, the whole constituting 
the phloem. Besides the ducts upon the inside of the 
cambium tracheids, wood fibers and wood parenchyma 
may develop, forming the xylem. The xylem and 
phloem, together with the intervening cambium, form a 
group constituting what is known as a fibrovascular bun- 
dle. Another kind of cells, which are nearly isodia- 
metric or radially elongated, also arise from the cam- 
bium and form radial rows, which are known as the 
medullary rays. The latter separate the fibrovascular 
bundles, and the number of cells, both as regards the 
width and depth of the rows, is characteristic for certain 
genera. 
At the same time that the cambium is developing 
xylem and phloem, another meristem, known as the 
phellogen, arises within the endodermis, which produces 
upon the inside parenchyma, known as the secondary 
