FLOWERS 



129 



->loc 



119. Symmetry of the flower. Except in a comparatively 

 small number of cases (of which the familiar canna, or Indian 

 shot, is a good example) the perianth usually shows some 

 kind of symmetry. Either (as in fig. 102) the parts are ar- 

 ranged in a radial fashion, like the points of a star, or else 

 they have corresponding halves 



in one plane to the right and 

 left of an axial line, as in pan- 

 sies and violets (fig. 106), in the 

 most familiar flowers of the Pea 

 family, and in those of the Mint 

 family. Flowers with radial 

 symmetry are said to be actino- 

 morphic, and those with corre- 

 sponding halves are said to 

 have bilateral symmetry, or to 

 be zygomorphic. 



Sometimes, as in the Milk- 

 weed family, the corolla has 

 extraordinary forms which fa- 

 cilitate seed production by the 

 aid of insect visitors (fig. 134). 



120. Parts of the stamen ; 



union of stamens. Many stamens g rains ; B > an older sta s e ' in which the 



. ., ... 1,1 pollen grams (p) are fully formed and 



have a form Similar to that shown each pair of locules is uniting to form a 



in figure 1 7, A, consisting of a P llen ** c > which wil1 8 P 1!t OP and dis - 



, . , .. , - charge along the line of dehiscence (d). 



rather slender stalk (the filament) A f ter Baillon and Luerssen 



which bears a stouter structure, 



the anther. Anthers which have no filaments are said to be 

 sessile. Within the anther the stamen produces a powdery or 

 pasty substance known as pollen, which, when magnified, is 

 seen to consist of minute grains (fig. 117). These differ 

 greatly in form and markings in different families of plants. 

 The mode of origin of the pollen sacs in which pollen is pro- 

 duced is partially shown in figure 108. When the sacs are 

 fully mature, they open and allow their contents to escape. 



FIG. 108. Diagrams to show struc- 

 ture of an anther 



A, younger stage, with four cham- 

 bers, or locules (loc), containing pollen 

 mother cells dividing to form pollen 



