234 MORPHOLOGY OP ANGIOSPERMS 



being not merely a protecting organ, but immensely varied in 

 form, texture, and color to secure entomophily. In other words, 

 the conspicuous function of the perianth in the petaloideous 

 groups is here assumed by the spathe, and the flowers retain 

 for the most part the primitive character. 



There are many features of the Aroids that suggest the He- 

 lobiales, especially the Potainogetonaceae, so that Engler 

 inclines to the belief that they have been derived from that 

 stock. If this be true, they represent a strong terrestrial branch 

 from the aquatic Helobiales, that in tropical conditions has 

 become extremely varied in form and structure, and that has 

 assumed the erect, climbing, and epiphytic habits. It does not 

 seem probable that any other monocotyledonous alliance is asso- 

 ciated with these two in origin ; but the suggestion has been 

 made that from the Aroids the Dicotyledons, or at least some 

 of their phyla, may have been derived. One of the most prom- 

 ising fields of morphological research is among the tropical 

 Aroids. 



The Lemnaceae represent a distinct reduction series, being 

 Aroids adapted to the free-swimming habit, and remarkably 

 reduced in structure, Wolffia being the smallest known seed- 

 plant. 



The six great alliances just considered constitute the Spiral 

 series of Engler, with inconstant number of floral members, 

 with mostly no perianth or one not adapted to entomophily, 

 and with a striking development of sheathing leaves or bracts 

 in connection with the inflorescence or the individual flowers. 



The four remaining alliances constitute the Cyclic series, in 

 which the almost constant floral formula is perianth 3 + 3, 

 stamens 3 + 3, carpels 3 and forming a syncarpous pistil. The 

 two perianth sets may be variously modified, but there runs 

 through the series an increasing specialization of the perianth 

 for entomophily, which reaches its extreme expression in the 

 Orchidaceae. As a consequence, the perianth rather than 

 bracts becomes the conspicuous floral feature. The pentacy- 

 clic trimerous habit having become established, the cyclic groups 

 have largely differentiated in the direction of a conspicuous 

 perianth, epigyny, and zygomorphy. The number of species 

 involved is so great that onlv the broadest outlines can be con- 

 sidered. 



