400 
AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
common in hypogynous flowers. The last is among the characters used by 
Planchon (15) in distributing the 16 species of Ulmus into three divisions. 
Celtis has perianth parts distinct to the base or nearly so. The remain- 
ing genera of the Ulmaceae, eleven in number, have perianth parts similar to 
those of Celtis (2). In the Ulmaceae the receptacle is limited, then, to the 
pedicel of the flower, and in Ulmus, coalescence and adnation have taken 
place in the perianth parts and stamens. Also, the fact that the single 
whorl of normal vascular bundles to the perianth and to the stamens, 
respectively, is accompanied by whorls of abortive bundles which alternate 
apparently with these, enforces the conclusion that along with the coales- 
cence and adnation, there has been reduction in these two sets of organs. 
This reduction consists of the loss of an inner whorl in each. Reduction 
occurs not only in the number of whorls but also in the number of organs 
within a whorl. No constant number exists in the floral whorls of any of the 
elms. Greater variation occurs in those species having the greatest number 
of organs present per whorl; e.g., in U. americana, as described on page 390, 
there are 9 to 7 perianth lobes and 9 to 5 stamens. The cause of the lack 
of floral symmetry in a species is due to the development of a perianth lobe 
without its accompanying stamen. This is usually the stamen to one or the 
other of the posterior-lateral lobes. However, just as often, a stamen 
develops without an accompanying perianth lobe. The number ranges from 
9 in Ulmus americana to 4 in U. campestris, and is more or less inconstant 
in all species. On the basis of inflorescence (which shows in Ulmus stages 
in reduction), the species with more floral parts are more primitive than 
those with fewer. Although the gynoecium of Ulmaceae is dimerous, from 
the presence of abortive strands to suppressed carpels it has suffered reduc- 
tion. Such organized tissue regions suggesting bundles were discovered 
in Ulmus americana. These bundles appear some distance above those to 
the stamens, on a level from which the strands to the carpels can be followed. 
Ulmus possesses spirally arranged parts (PI. XV, figs. 7-14), though the 
other genera studied are cyclic. The spiral arrangement is most conspicuous 
in the species with the greatest numbers of stamens and perianth parts and 
becomes less conspicuous in those elms in which the floral characters grade 
into those of the Moraceae which are tetramerous and cylic. The spiral 
arrangement is an important phyletic character, but by reduction in the 
number of organs and in the floral axis, it has become nearly obscure. 
The alterations in the posterior part of the flower over those of the 
anterior part by modification in the relation of organs to each other, and 
by the suppression of organs, form a true zygomorphy. This character is 
perhaps the result of aggregation (23), and possibly an adaptation to insect 
visitation. To be sure, very few species in the Urticales are known to be 
visited by insects, yet zygomorphism may be a character persisting from 
an earlier time when insect visitation was the common occurrence. There is 
a possibility that zygomorphism as a specialized character and as a character 
