1897] PHYLOGENY AND TAXONOMY OF THE ANGIOSPERMS 161 
passing that these present similar diversity of form and func- 
tion in both sub-classes, and that from this fact we may infer 
the close relationship, if not the common origin of the mono- 
cotyledons and dicotyledons. 
The tissue systems present no constant differences in the 
boundary and fundamental systems. Possibly the surface 
appendages (trichomes) reach a higher development in some 
dicotyledons than in any monocotyledons. The skeletal system 
shows some well marked differences. In monocotyledons the 
fibrovascular bundles are typically separate, while in dicotyle- 
dons they are typically united with one another. In the former 
_ each bundle is complete in itself, and is often sharply defined 
by a bounding layer of cells, while in the latter the bundles 
form parts of an aggregation in which the limits of the individ- 
ual bundles are often indistinguishable. The shorter life of the 
bundle in the monocotyledons contrasts sharply with its longer 
life in most dicotyledons, sometimes reaching hundreds of years, 
as in the long-lived oaks and chestnuts. And yet these differ- 
ences, sufficiently constant to characterize the sub-classes, are 
notinvariable. There are skeletal systems in some dicotyledons 
whose bundles are separate, short lived, and incapable of con- 
tinued growth, showing again the close relationship of the two 
sub-classes. 
The organs of the plant body present great diversity, and 
their morphology has long been the subject of much study by 
many investigators. They may be reduced to the following 
types: roots, stems, foliage leaves, flower leaves, pollen leaves, 
ovule leaves. 
Roots —The young roots of monocotyledons are structurally 
simpler than those of dicotyledons. They rarely increase much 
in thickness or endure for any great length of time, and are 
usually unbranched. They contain a single central fibrovascular 
bundle. The roots of dicotyledons when young containa single 
central bundle, but they generally develop several collateral 
bundles, and are thus able to increase in thickness and to endure 
for an indefinite time. They are commonly branched again and 
