SPERMATOPHYTES 281 
nized, which are grouped into eight alliances, whose characters and 
sequence are quite evident. 
The first three alliances are called the pentacydic and isocarpic alli- 
ances. The former term means that there are five cycles of floral 
members, the stamens being in two cycles; the latter term means that 
the number of carpels is the same as that of other cycles. The floral 
formula which expressses both of these facts is as follows: sepals 5, 
petals 5, stamens 5 + 5, carpels 5 (syncarpous). These pentacyclic 
Sympetalae are most nearly related to the Archichlamydeae, and in 
fact contain some polypetalous forms. They are not very numerous, 
including only about 3500 species. 
1. Ericales (6 families, 1700 species). The heaths constitute the 
dominant family (Ericaceae), very characteristic of northern latitudes. 
Some of the forms are polypetalous, but they are so related to sym- 
petalous forms that they cannot be separated from them ; and in some 
of the sympetalous forms the stamens are free from the corolla. One 
of the features of the alliance is the characteristic dehiscence of the 
anthers, which is by means of terminal openings in the tubular prolonga- 
tions of the pollen sacs. 
2. Primulales (3 families, 850 species). Two features of this alli- 
ance, of which the primroses (Primulaceae) are representatives, are 
the opposite stamens and free central placenta. The five stamens 
are opposite the five petals, instead of alternate with them, as is usual; 
but this is explained when it is discovered that the outer cycle of stamens 
is abortive, being represented by rudiments called staminodia. The 
axis of the flower extends into the ovary cavity like a central column, 
and upon it the cauline ovules are developed, a condition which was 
formerly called free central placentalion. 
3. Ebenales (4 families, 900 spec es). These are mostly tropical 
shrubs and trees, represented in our flora by the persimmon (Diospyros), 
As the name suggests, the characteristic family is the ebony family 
(Ebenaceae). It is a curious mixture of primitive and advanced char- 
acters, with frequent lapses into indefinite numbers, especially of stamens. 
The remaining alliances are tetracyclic and anisocarpic. This means 
that there are usually only four floral cycles, and that the number of 
carpels is not equal to that of the other cycles. The general floral formula 
is as follows: sepals 5, petals 5, stamens 5, carpels 2 (syncarpous). In the 
more primitive alliances the carpels fluctuate between five and two, often 
being four or three. The five tetracyclic alliances fall naturally into 
