230 MORPHOLOGY OF AXGIOSPERMS 



ent, but the line -as a whole presents almost a complete series 

 from the simplest floral structure to one of the most highly 

 developed. 



The series of floral changes may be broadly indicated as 

 follows. In Potamogetonaceae and ^Naiadaceae there is no peri- 

 anth, and the stamens and carpels are indefinite in number ; in 

 Juncaginaceae a bract-like perianth is present, there is a dis- 

 tinct tendency toward the trimerous habit, and syncarpy may 

 occur; in Alismaceae the perianth is differentiated into calyx 

 and corolla, and the trimerous tendency is very clear, though 

 the carpels are usually indefinite in number; in Hydrocharita- 

 ceae, in addition to a differentiated perianth and a strong ex- 

 pression of the trimerous tendency (although the stamens and 

 carpels are often indefinite in number), the flowers are epigy- 

 nous. The plants are chiefly anemophilous or hydrophilous, 

 but the appearance of a differentiated perianth in the Alisma- 

 ceae is probably associated with a certain amount of ento- 

 mophily. 



Heliobales, therefore, begin with as great simplicity of 

 floral structure as do the Pandanales, but they have advanced 

 much further in floral development. That such an extensive 

 line comprises so few species is probably associated with the 

 uniformity of aquatic conditions. In the whole series, how- 

 ever, there is no distinct settling into a complete trimerous 

 habit, which is intimated rather than established. 



III. GLTJMALES.* In this alliance are the two great fami- 

 lies Gramineae and Cyperaceae, the former including about 

 351 genera and 4,700 species, the latter 76 genera and about 

 2,300 species. In point of species this is one of the greatest of 

 angiospermous alliances, and in display of individuals it is un- 

 questionably the greatest. The common features of the two 

 families are the absence of a perianth, the protection of the 

 flowers by special bracts, the fluctuating of the stamens between 

 one and many, the solitary carpel, and anemophily. It is not 

 probable that the two families are related to one another genet- 

 ically, but they represent approximately the same stage of floral 

 development. 



The peculiar features of the bract-protection, as contrasted 



* GLUMIFLORAE of Engler. 



