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 hypo- 

 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 i2 cm , and the diameter of the hypocotyl may 

 have increased to about i cm . After a long period (which in the 

 greenhouse was about four weeks in case the micropylar end was 

 cut away, but many months if it was not) the first leaf appears between 

 the cleft of the cotyledons, in direct continuation of the axis. As the 

 natural position of the seed during germination is horizontal, and the 

 hypocotyl bends down approximately at right angles, the leaf pushes 

 forth where the cotyledons make a sharp bend. The cotvledons 

 remain in the seed, and in seedlings three or four years old the seed 

 may be seen still attached to the plant. At this time also lateral 

 roots appear in four rows along the primary root, corresponding to 

 the four protoxylem groups. 



A second leaf does not appear until perhaps a year later, and 

 about another year elapses before a third appears. A plant of this 



