260 MOKI'MOLOGY 



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), 



