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 placentation. 



3. Ebenales (4 families, 900 species). 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 



