368 BOTANICAL GAZETTE [NOVEMBER 
cambium stretching more than half-way across the bundle, the 
boundary being marked by a thicker-walled tissue, the protophloem. 
Higher up in the bundle the centrifugal procambium decreases 
proportionally as the protoxylem moves outward. Figs. 4, a and 21 
show the situation more clearly; both are taken from very young 
primordia, the latter much younger and higher up than the former. 
In fig. 21 the xylem elements are approximately in the center of the 
bundle, but there is still considerable procambium between them and 
the protophloem. The cambium appears to be developed later, just 
inside the protophloem, and develops tissue actively toward the 
phloem side only. 
THE SEEDLING 
EXTERNAL STRUCTURE.—When the seed of Dioon germinates, the 
hypocotyl pushes through the micropylar end of the seed, where there 
is at this time an area in the testa, about the diameter of the embryo, 
which softens readily and is easily penetrated by water. If left to 
itself, this process takes from a week to a month, and sometimes 
longer; but if the area referred to is cut away, the hypocotyl pushes 
through in a few days. During germination the whole of the hype 
cotyl and the lower part of the cotyledons lengthen. 
As soon as the hypocotyl reaches the soil a tap root is sent deep 
into the ground, and before any leaves appear the root may have 
penetrated the soil 8 to 12°", and the diameter of the hypocotyl may 
have increased to about 1°™, After a long period (which in Lp 
greenhouse was about four weeks in case the micropylar end was 
cut away,'but many months if it was not) the first leaf appears betwee? 
the cleft of the cotyledons, in direct continuation of the axis. As M7 
natural position of the seed during germination is horizontal, and wid 
hypocotyl bends down approximately at right angles, the leaf pushes 
forth where the cotyledons make a sharp bend. The cotyledons 
remain in the seed, and in seedlings three or four years old the 
may be seen still attached to the plant. At this time also lateral 
roots appear in four rows along the primary root, corresponding ad 
the four protoxylem groups. : 
A second leaf does not appear until perhaps a year later, an 
about another year elapses before a third appears. A pl oe 
