THE ROOT OF THE ANGIOSPERM 35 
formed kind of cellulose known as lignin. The cross 
walls have disappeared, and so these cells form long tubes 
known as vessels stretching throughout the root and 
stem. The largest cells of the phloem, on the other hand, 
are still lined with protoplasm, and the cross walls instead 
of disappearing have become thickened and perforated 
Fria. 20.—Transverse Section of the Root of the Bean. 
pl. Piliferous layer. c. Cortex. end. Endodermis. pe. Pericycle. 
aw. Xylem. phil. Phloem. p. Pith. (From Darwin’s “ Elements 
of Botany ”.) 
like sieves. The walls have undergone no change and 
still give the characteristic reactions of cellulose. These 
groups of phloem and xylem are arranged around a 
compact tissue known as the pith, composed of cells 
very similar to those of the cortex. They are separ- 
ated from each other by parenchymatous tissue known 
as conjunctive tissue. The first-formed elements of 
the xylem known as the protoxylem are towards the 
3 * 
