SEEDLING STRUCTURE IN THE LEGUMINOSJE. 105 
We can hardly avoid the conclusion that the tetrarchy found in slender 
seedlings is a reduced form of that found in large seedlings ; its preservation 
being in relation to the useful function performed by the intercotyledonary 
root-poles in supplementing the early plumular traces. 
REDUCTION OF THE NUMBER OF PROTOXYLEMS. 
While there exists an intimate relation between the plumular bundles and 
the intercotyledonary root-poles, it appears that the latter can readily be 
dispensed with. Their absence entails the production of a little more 
secondary xylem in the hypocotyl before root and leaf can be put into direct 
communication ; so that the diarch type is not at a very pronounced dis- 
advantage as compared with the tetrarch. That some advantage accrues 
from the possession of intercotyledonary xylems is clear from the occurrence 
of triarchy ; but we find so many cases in which, within narrow circles of 
affinity, one or both of the lateral root-poles may be present or absent or 
vestigial, that we can attribute to them no profound functional significance. 
If we assume, as there is good reason to do (p. 104), that the form of 
tetrarchy in slender seedlings is derived by reduction from tetrarchy 
in bulky seedlings, it follows that triarehy and diarehy are further 
stages in the reduetion. There seems to be no other explanation for the 
close association between tetrarchy, triarchy, and diarchy in the slender 
seedlings of the Trifolieze, &e., than that we have here a group of plants in 
an unstable condition with respect to vascular symmetry. The lateral coty- 
ledonary root xylems of the larger tetrarch forms are in the process of 
'anishing, perhaps in relation to the decrease in bulk of the seedling ; their 
final disappearance is, however, delayed by the somewhat useful function 
they subserve with regard to the plumular traces, but this secondary 
usefulness is too slight to have much influence in checking the evolution 
towards diarchy. 
Tetrarehy, triarchy, and diarchy may co-exist in a single seedling, e. g., in 
Ononis rotundifolia, Medicago turbinata (324-2), and Dorycnium hirsutum 
(2>3>4). These and many other cases in which only two of the types 
co-exist show that a decrease in the number of protoxylems may occur in 
some cases as we descend the axis, in other cases as we ascend it. A 
particular lateral xylem may be present in the root and absent in the 
cotyledons ; it may be present in the cotyledons but unrepresented in the 
root; or it may be present in neither cotyledons nor root, but may run a 
short isolated course in the hypocotyl. 
It thus appears that the reduction from tetrarchy may occur at either the 
top or the bottom of the hypocotyl, and in certain cases at both ends 
simultaneously. There seems, however, to be a distinct preference for the 
reduction to take place below rather than above. Out of 26 species of 
