436 Worsdell. — Origin and Meaning of Medullary 
The smaller strands of the abaxial row, alternating with the larger 
ones, are situated at a slightly shorter radial distance from the lacuna than 
the larger bundles ; this is , important as indicating that they represent 
a transitional series between the central medullary strands and the larger 
bundles of the abaxial row. 
The vascular structure of this genus is instructive. We have seen that 
medullary strands occur in the stem of some species, but not in that of 
others ; and in the stem of certain individuals of the same species, but are 
absent in that of other individuals. Moreover, in the latter cases they may 
occur in certain internodes of the same stem but are absent in others. 
These facts clearly point to the conclusion that the medullary strands are 
vestigial (i. e. ancestral) structures which are on the verge of extinction in 
the stem. 
On the other hand, the petioles of all species examined showed the 
very obvious presence of medullary strands in larger or smaller numbers ; 
but they are always of extremely small size and on the verge of extinction, 
hence no longer of any use to the plant. They represent an ancestral 
feature which once prevailed in both stem and leaf of all species. 
Picris hieracioides, L. 
Both wild and cultivated specimens were investigated. 
Stem 1. 
Medullary strands are absent from the internodes and from the 
branchless nodes. But in the nodes at which branches are given off they 
occur, as in the stem of Crepis biennis , to which genus Picris is closely 
allied. 
At one node -a bundle passed into the stem-ring from the vascular 
ring of the branch and took up a position in the inner entrance of the 
branch-bay, where it divided into two ; these two bundles then gradually 
died out at a higher level in the node as the stem-ring closed up behind 
the outgoing branch-ring. At another node two medullary bundles arose 
de novo at the bay-entrance, fused together, and died out in situ while the 
ring closed up. At another node two bundles arose de novo , one of which 
divided; of these three one had apparently died out in the next section ; 
the remaining two also died out before the ring had closed up. At two 
other nodes the same events occurred. All the above-mentioned nodes are 
at successively higher levels on the stem. 
Stan 2. 
At all the nodes in the lower part of the axis where no axillary 
branches, but only small buds, occur no medullary strands are to be found. 
In the lowest of the branch-nodes, however, medullary strands occur and 
