188 BOTANICAL GAZETTE [MARCH 
smaller size. These protoxylem elements are spiral vessels; they 
always occur at the inner margin of the xylem, that is, in the 
endarch position. The protophloem, consisting of 3 or 4 cells, 
develops at the opposite limit of the strand; the later developed 
phloem of 6-10 cells is arranged in 2 or 3 layers. One or two layers 
of parenchyma separate the xylem and phloem in a mature bundle 
(fig. 15). 
In the mature strands of the blade, and, to a less extent, of 
the petiole, a definite 
bundle sheath is devel- 
oped by the thickening 
of the walls of 2 or 3 
layers of parenchyma 
surrounding the vascu- 
lar elements (fig. 15). 
The thickening material 
is cellulose and the walls 
are pitted as in similar 
tissues of the stem and 
root. This bundle 
sheath is separated from 
the protoxylem by 2 or 
3 cells of parenchyma, 
but borders directly 
upon the protophloem. 
As a consequence of 
growth within the 
bundle sheath, the protophloem in a mature strand is crushed 
against and between the cells of the sheath. 
Fic. 15.—Detail of a leaf strand: pph, proto- 
phloem; ph, metaphloem; px, protoxylem; x 236. 
The spike 
As stated above, the strands of the leaf with xylem directed 
abaxially form the vascular supply of the spike. At the base of 
the peduncle these are 4-6 in number; they continue with occa- 
sional anastomosing and fusion through the median portion of the 
spike. At the base of the fertile portion of the spike, the strands 
at the margin of the median region run immediately at the base 
