170 de Fraine. — The Seedling Structure of certain Cactaceae . 
On this point the seedling structure of the Cactaceae can throw some 
light. In general the upper part of the hypocotyl of these seedlings is 
occupied by two ‘ double ’ bundles, which may have been formed in one of 
the following ways :— 
1. By the bifurcation of the cotyledonary bundle, e. g. Echinopsis 
multiplex. 
2. By the fusion of separate vascular strands. 
The strands which fuse may be :— 
(a) The two separate bundles present in a cotyledon, e. g. Echin¬ 
opsis Lager mannii, Cereusperuvianus , and Echinocereus Ehren- 
hergii. 
(b) The cotyledon-bundle and its tubercle-bundle, e. g. Echinocactus 
Ottonis , Series 3. 
(c) The fused cotyledon- and tubercle-bundles to form one half of 
the double bundle, the other half being similarly produced, e. g. 
Echinocactus Ottonis , Series 1, Mamillaria vmlticcps. 
(d) The fused cotyledon- and tubercle-bundles to form one half of 
the double bundle, the other half being formed by the fusion 
of the other two cotyledonary strands, e. g. Echinopsis tubiflora. 
(c) The epicotyledonary strands only, e. g. Mamillaria pusilla. 
Further, in Mamillaria mis sour iensis the ‘double’ bundle may be 
produced either by bifurcation of a single strand or by the fusion of the 
separate ones ; while in Mamillaria centricirrha one ‘ double ’ bundle is 
derived by the splitting of the cotyledon-trace, and the other by the inter¬ 
action of a tubercle- and a cotyledon-strand. 
A consideration of these various methods of formation of the ‘ double ’ 
bundle in the Cactaceae points to the fact that it is unsafe to formulate any 
theory on the homology of such variable structures; 1 for they would 
appear to be nothing more than an arrangement of the vascular elements 
in such a way that the change from stem to root structure may take place 
as conveniently as possible ; they would certainly not appear to have any 
definite morphological value. If then the ‘double’ bundle, which appears 
to be a fairly constant feature of the cotyledon of Monocotyledons, is not 
necessarily the homologue of two separate bundles, that is, if the ‘double’ 
bundle has no definite morphological significance, it is quite possible to 
account for the distinction between the vascular symmetry of the cotyledon 
and of the ‘ first leaf ’ in the Monocotyledons. Thus, supposing that the 
ancestor of the group was a seedling with two equivalent cotyledons, 
adaptation to a geophilous habit led to these cotyledons assuming different 
functions; one serving as an organ for the absorption of the endosperm, 
the other retaining its assimilating function. Since the need for economizing 
1 Thomas, E. N.: A Theory of the Double Leaf-trace founded on Seedling Structure. New 
Phyt., vi, p. 88. 
