THE FLOWER AND FRUIT 89 



tive variations in the number, size, shape, and degree of divergence 

 of these parts. On the basis of this concept, it is possible to inter- 

 pret the great variety of floral structures, and to interrelate them 

 in a logical manner. 



The sepals collectively constitute the calyx, forming the outer- 

 most spiral or cycle of the floral axis; and the sepal often resembles 

 a vegetative leaf in structure and venation. They are usually 

 chlorophyllose and photosynthetic; but their primary function, 

 at least in the early ontogeny of many flowers, is that of protec- 

 tion. In the bud stage, they form an enveloping structure which 

 overarches and encloses the other floral parts prior to anthesis. 

 Other specialized functions are attraction of insects and the dis- 

 semination of the fruit. In some instances, the calyx is entirely 

 lacking. 



The petals, collectively termed the corolla, comprise the second 

 or inner set of the perianth. They may occur in one or more 

 spirals or cycles and are often white or some color other than 

 green. Their function is primarily that of attraction of insects, 

 but they also serve as a protection to the stamens and carpels. In 

 this connection, they may be specialized as to their shape, position, 

 or habit of opening and closing. In some floral types (Anemone), 

 the petals are lacking; and the sepals are brightly colored, or 

 there may be brilliant leafy bracts (Poinsettia). In wind-polli- 

 nated cereals, the petals, like the sepals, are either much reduced 

 or absent. 



The stamen or microsporophyll consists of a stalk or filament 

 and a terminal anther comprised of one to several (usually four) 

 microsporangia or pollen sacs. During the development of the micro- 

 sporangia, and following reduction division, microspores and even- 

 tually pollen grains (microgametophytes) are formed. Com- 

 monly, the wall between adjacent microsporangia is broken; so 

 that, in the mature stamen, the anther consists of two locules or 

 pollen chambers instead of four separate microsporangia. These 

 pollen sacs dehisce in various ways at maturity, and the pollen 

 may be exposed or liberated through terminal chinks or pores, or 

 by means of a splitting of the anther wall. 



The carpels or megasporophylls constitute the central or termi- 

 nal set of floral structures, and are referred to singly or collectively 

 as a pistil, or the pistils. The pistil may be simple consisting of 

 a single carpel, or compound if it is made up of two or more carpels. 



