1904] CHRY SLER—CENTRAL CYLINDER 181 
which may be arranged in a circle or scattered; (4) only collateral 
strands are found in the leaves. Hence amphivasal strands are to 
be regarded as cenogenetic structures. 
The observations recorded in this paper seem strongly to support 
the statement made by JErrREy (8) that neither the medullary course 
of the bundles nor their amphivasal nature are primitive features, 
but that they appear at a more or less late stage, and that they serve 
to distinguish monocotyledons from other groups. The plan of the 
young stele, e. g., Smilacina, bears a close resemblance to that of a 
dicotyledon, and differs from the older stele of a dicotyledon only in 
the absence of cambium. The resemblance between the two groups 
is further shown by the occurrence of medullary strands in several 
dicotyledonous families, e. g., Nymphaeaceae, and in the older sub- 
teranean stem of Ranunculus acris (6, p. 20); also by the occurrence 
of amphivasal strands in the mature tissues of such plants as Rheum 
and Campanula. These considerations lead to the conclusion that 
the monocotyledons are not an ancient group, but that they have 
branched off from the dicotyledons, or that both groups have sprung 
from a parent stock which resembled the modern dicotyledons more | 
closely than it did the monocotyledons. . 
CONCLUSIONS. 
1. The members of the Araceae and Liliaceae have primitively a 
collateral tubular central cylinder, or ectophloic siphonostele, derived 
from a protostele, and interrupted by gaps above the points of exit 
of the foliar traces; through these gaps the external and internal 
phloeotermas communicate; the intrastelar parenchyma is to be 
regarded as having the same origin as the cortex, 7. e., both cortex 
and medulla are portions of the fundamental or ground tissue. 
2. This primitive condition becomes altered (1) by degeneration 
of either the internal phloeoterma or both the internal and external 
Phloeotermas; (2) by the assumption of a medullary course by some 
vascular strands, with which leaf traces are connected; hence the 
scattered arrangement of bundles is to be regarded as a cenogenetic 
character. 
3- The amphivasal concentric strands are not a_palingenetic 
feature, for they are derived from collateral strands and do not occur 
in the base of the seedling nor in the leaves and floral axes. 
* 
