DICOTYLEDONES 



411 



upon the leaves of xerophytes act as a screen against the too intense 

 light rays. 



Flowers of Dicotyledons 



Much the same range of structure is found in the flowers of 

 Dicotyledons that obtains among the Monocotyledons. Some of 

 the lower forms are destitute of any proper floral envelopes, and 

 in rare instances e.g. Ascarina may consist of but a single 

 carpel or stamen. In these simplest Dicotyledons, stamens and 

 carpels are frequently in different flowers (Quercus), or even upon 

 different plants (Morus, Cannabis). While some of these simple 

 flowers may be reduced forms, most of them cannot be so regarded, 

 and must be considered to be 

 primitive types. B 



b' 



FIG. 390. A, B, Peperomia blanda. 

 A, portion of the spike, showing the 

 arrangement of the simple flowers 

 (X<>). B, single flower, more en- 

 larged; b, the subtending bract; 

 ?, carpel; (,, stamen. C, P. sub- 

 rotunda. Median section of flower 

 (X 40) ; car, carpel. 



FIG. 391. A, Oxalis sp. Actinomorphic 

 choripetalous flower. B, Spirsea s;>. 

 Section of the actinomorphic flower ; all 

 the parts separate; car, carpels. C, 

 Lamium album. Sympetalous, zygomor- 

 phic corolla. D, Veronica sp. Showing 

 zygomorphic sympetaly, and reduction 

 of stamens to two. 



In the greater number of Dicotyledons the flowers possess well- 

 developed floral envelopes, which are differentiated into calyx and 

 corolla. Sepals and petals are most commonly four or five in 

 number, although there are numerous exceptions. In one group 

 (Ranales) the number of parts is often indefinite (Magnolia, Caly- 

 canthus), and this is true of the stamens and carpels as well. These 

 Ranales usually are " apocarpous," i.e. have the carpels separate, and 



