260 MORPHOLOGY 
down, and each pair becomes one continuous spore chamber or sac, 
called the pollen sac (fig. 580). The pollen sac of angiosperms, there- 
fore, is usually composed of two coalesced sporangia. The dehiscence 
of the pollen sacs, in the discharge of the spores (pollen grains), is most 
commonly by a longitudinal slit, developed where the two coalesced 
sporangia join (figs. 572, 580); but sometimes they open by terminal 
slits or pores (fig. 573), or by openings in tubular prolongations of the 
pollen sacs (fig. 574), or sometimes by hinged valves. 
CARPEL 
General character. The carpel is a megasporophyll, and though 
often it does not produce the megasporangium (ovule), it always in- 
closes it. Ovules, on account of their relation to its tip, frequently 
arise from the axis ; so that ovules among angiosperms are both cauline 
and foliar in origin. The carpel is usually organized into two dis- 
tinct regions : the ovary, in which the ovules occur ; and the style, 
usually a more or less elongated and stalklike region arising from the 
top of the ovary (figs. 566-568). Upon the style, usually at its tip, some- 
times along one side, there is exposed a special tissue that receives the 
pollen, known as the stigma. This stigma is the exposed part of a tissue 
which extends through the style (sometimes lining a stylar canal) and 
along the wall of the ovarian cavity, and forms the nutritive path of 
the pollen tubes on their way from the stigma to the ovules. This 
tissue in the style has been called conducting tissue, and in the ovarian 
cavity the placenta. 
In cases of syncarpy, two or more carpels are organized together, 
forming a single ovary (fig. 567), and often a single style (fig, 568). In 
such cases the ovary may contain as many chambers as there are carpels, 
or there may be only one chamber. Since carpels may be organized 
singly or collectively, it is convenient to have a general term that can be 
applied to either kind of carpel organization, and that term is pistil. 
A simple pistil is one composed of a single carpel (fig. 566); while a 
compound pistil is composed of more than one carpel (fig. 567), and may 
contain as many chambers as there are carpels in the organization, or it 
may contain a single chamber. 
Ovule. The ovule may arise from any free surface within the cavity 
of the ovary; and since this free surface involves both the carpels and 
the tip of the axis (sometimes prolonged into the cavity of the ovary), 
