160 Sargant. — The Reconstruction of a Race of 
perhaps accompanied or even replaced, by that of the hypocotyl. Then 
follows the lengthening of the primary root, and finally that of the plumule, 
or of the first leaves. This sequence is the rule in Monocotyledons and 
Dicotyledons alike, but, of course, exceptions are found in both classes. 
Seedlings adapt themselves in this as in other respects to the demands of 
their environment. 
In every seedling there is a period — longer in some species, shorter in 
others — during which it consists of cotyledons, hypocotyl, and primary root. 
The plumular bud is of course present, but neither its axis nor the petioles 
of its leaves have begun to elongate. It is insignificant compared to the 
cotyledons. During this period the vascular tissue within the seedling 
becomes well-defined for the first time. Lignification of the xylem sets in 
— a change of great importance to the microscopist, since it permits him to 
pick out the first xylem elements with certainty from other constituents of 
the bundle. In many slow-growing seedlings the vascular skeleton of the 
cotyledons, hypocotyl, and primary root, is fairly complete before the 
plumular traces are well differentiated. In such forms this vascular skeleton 
is often found to be characteristic of the genus, or even of a larger group. 
In other words, the skeleton is formed on the same type throughout the 
genus or the order. Simple as this skeleton necessarily is, more variation 
in detail is possible within it than might be expected. The number of 
traces furnished by each cotyledon, their behaviour in the hypocotyl during 
the transition to a root-structure, the symmetry of the primary root, are all 
variable characters. Very well-marked types of vascular symmetry can be 
distinguished, and some of these are clearly derived from others. Thus 
extended observations show some types to be more primitive than others. 
The seedlings of Monocotyledons and Dicotyledons have been exa- 
mined of late years with a phylogenetic aim. General conclusions have 
been drawn from comparison of the several types of vascular symmetry 
found within each class. If such investigations should show that there is 
a primitive type in each class, we may fairly suppose it to represent the 
seedling skeleton of the ancestral Dicotyledon or the ancestral Monocoty- 
ledon. But of course the primitive or ancestral Dicotyledon and the 
primitive Monocotyledon are nearer to their common ancestor than living 
Dicotyledons and living Monocotyledons are. Therefore the primitive 
forms in either class should resemble each other more closely than living 
Dicotyledons resemble living Monocotyledons. 
Some fairly extensive observations on the vascular symmetry of 
seedling Monocotyledons have shown that one particular type of mono- 
cotylar symmetry may be considered relatively primitive (Sargant, 72). 
A certain dicotylar type has also been picked out as relatively primitive 
by other observers, partly by comparison of Dicotyledonous seedlings with 
each other, but chiefly as the result of comparison with Gymnosperms 
