226 A TEXT-BOOK OF BOTANY 
visible from the outside, looking like two sacs, called pol- 
len sacs. Ordinarily, therefore, the Angiosperm stamen 
is said to have two pollen sacs. 
In most cases the pollen sacs must open so that the 
pollen may escape, and the method of opening differs in 
different flowers. By far the 
most common way is for each 
pollen sac to split open length- 
wise, and this line of splitting 
is usually plainly seen on the 
surface of the unopened sac 
(Fig. 210, A). In some cases, 
however, each pollen sac opens 
at the top, either by a short 
slit or by a pore-like opening 
(Fig. 213, A and B); and in 
Fic. 214.— Section of the flower of an 
Althea, showing sepals (a), petals some cases, aS among the 
(b), tube of stamens (c) enclosing heaths, this pore-like open- 
style (d), and ovules (e).—After . . 
Bere and Scumrpr. ing may be extended into a 
more or less prominent tube 
(Fig. 213, C). There are still other special methods of 
opening pollen sacs, but they are comparatively rare. 
In sympetalous corollas it is most common for the 
stamens to appear fastened to the tube of the corolla (Fig. 
208, B), and this condition is usually described as ‘stamens 
inserted on the tube of the corolla.” Stamens may also 
appear united, forming a tube, as in mallows (hollyhock, 
etc.) (Fig. 214); or they may be in two sets, as in the sweet 
pea, in which nine of the stamens appear united and the 
tenth one is free (Figs. 241 and 283). 
135. Carpels.—It has been noted that carpels are the 
sporophylls that bear the peculiar sporangia called ovules 
(§ 123). There is a striking difference, however, between 
the carpels of Gymnosperms and Angiosperms, a difference 
that gives names to the two groups. In Gymnosperms the 
