192 



BOTANY: PRINCIPLES AND PROBLEMS 



turgidity of the cells rather than by skeletal tissues. In certain 

 cases, however, notably in the grasses and allied families, the 

 perianth segments have become hard, dry, and chaffy. 



Any one, or more than one, of the floral circles may sometimes 

 be absent. If both calyx and corolla are missing the flower is 



Fig. 105. — Cutkias from a male ijl.int dI uiUow [.Siili.i, .-Lowing the masses 

 of stamens. 



Fig. 106. — Catkins from a female plant of willow. The prominent pistils 

 can readily be seen. 



said to be naked. If it is either the stamens or the pistil which is 

 absent, the flower is unisexual and is called either "male" or 

 "female" according to the structures which it possesses. If both 

 male and female flowers are distinct from one another but are on 

 the same plant (as in corn, birch and many others) the plants are 

 said to be monoecious; if the two sexes are on separate plants (as 

 in the willow. Figs. 105 and 106) the species is termed dioecious. 

 In studying the evolution of floral parts, evidence has been 

 obtained that the sepals, petals, and probably the stamens and 



