372 



PLANT MORPHOLOGY 



a reduction series that begins with the gymnosperms and woody dicotyle- 

 dons and passes through the herbaceous dicotyledons, where every inter- 

 mediate condition is seen. This reduction series indicates the general 

 trend of evolution as it seems to have taken place in the spermatophytes. 



Fig. 317. Cross section of an amphivasal bundle from the rhizome of sweet flag {Acorns 

 calamus), a monocotyledon, showing the xylem completely surrounding the phloem, X 500. 



The Flower 



A strobilus is a group of sporophylls borne on a more or less elongated 

 axis. A flower is essentially a strobilus in which the sporophylls (stamens 

 and carpels) are usually borne on a shortened axis (receptacle) and are 

 usually surrounded by a perianth. This distinction is untenable, how- 

 ever, because some flowers have an elongated receptacle and some have 

 no perianth. For convenience, any organization of sporophylls may be 

 designated as a strobilus in pteridophytes and gymnosperms and as a 

 flower in angiosperms. Such a distinction is arbitrary. A flower and a 

 strobilus are morphologically equivalent structures. 



Practically all gymnosperms, except the Bennettitales, have monospo- 

 rangiate strobili, while most angiosperms have bisporangiate ("perfect") 

 flowers. In many cases the monosporangiate ("imperfect") condition 

 has arisen by the suppression of stamens in the one kind of flower and of 

 carpels in the other, the reduced organs often being represented by 



