626 
PHANEROGAMS. 
formed into petaloid staminodes. It has already been pointed out how the flowers 
of Gramineae and Orchideae can be traced back to the trimerous pentacyclic type ; 
the theoretical diagrams here given (Figs. 426 — 433) will answer the same purpose 
for some of the other more important families. 
If the pentacyclic flower with the formula S^^P^Si^j^^C^^^^^) is considered as 
the typical one for Monocotyledons, it will be seen that the great majority of 
families the number of whose parts deviates from this type do this only by 
the suppression of single members or of whole whorls, the typical position of 
those that still remain with respect to one another not being disturbed. The 
Fig. 431. — Diagram of AlisniaceK ; A Butomns, B Alisma. FIG. 432. — Diagram of Triglochin (Juncagineje). 
variety in the forms of flowers in this class is therefore brought about almost 
entirely by abortion ^ ; and it is not uncommon for abortion to be carried to such 
an extent in Monocotyledons that nothing is left at last of the whole flower but 
a single naked ovary or a single stamen, as happens frequently in Aroideae. In 
these cases a similar explanation of the relationships of the parts of the flower is 
rendered possible and even evident by the occurrence of flowers with the actual 
typical structure, and by a complete series of transitions caused by partial abortion. 
It is especially in small closely crowded flowers, as those of Spadiciflorae, Glumi- 
florae, &c., that so great a reduction of the typical number of members is observed ; 
FJG. 433. — Diagram of Gymiiostachys (Aroideae), after Payer. 
while in larger and more isolated flowers the number of members in each whorl 
is usually complete or even excessive (as Butomiis and Hydrocharis), and deviations 
usually result from petals (or petaloid staminodes) being formed in the place of 
fertile stamens [e.g. Scitamineae). With reference to the abortion which is often 
carried to so great an extent in small flowers, it may in certain cases even be 
doubtful whether in an assemblage of stamens and carpels we have a single flower 
or an inflorescence consisting of several flowers reduced to a very simple state by 
abortion, as for example in Lemna. 
^ Compare what was said on Abortion at p. 222 and in the Introduction to Angiosperms. 
