202 OUTLINES OF PLANT LIFE. 



called the placenta. The placenta in angiosperms is com- 

 monly located at the united edges of the carpel or car- 

 pels. If the carpels are united into a compound pistil, the 

 placentas will be either isolated, as ridges upon the inner 

 face of the wall of the ovulary (fig. 158), or aggregated at its 

 center (fig. 157). Occasionally the ovules arise upon the 

 entire inner face of the carpels, as in the gentians. 



287. Stamens. — A stamen is a leaf of the seed plants 

 which bears the pollen sacs. The flowers whose essential 

 organs are all stamens are said to be staminate. Rarely a 

 single stamen constitutes a flower. Except for the crowd- 

 ing, the stamens are arranged like all the other leaves of the 

 plant, arising on the axis alternately, or in one or more 

 circles. The stamens exhibit great diversity of form and size. 

 Each usually consists of two parts, a stalk, called the 

 filament, bearing an enlarged portion, called the anther (st, 

 fig. 66). 



The anther is usually larger than the filament and com- 

 monly two-lobed, having the sporangia located in the thicker 

 parts. 



288. Spore cases. — The anther bears from 1-12 pollen 

 sacs (spore cases) upon its surface, or wholly or partly sunk 

 in its tissues. In most anthers the pollen sacs are either two or 

 four (fig. 161). When there are four they are often paired, and 

 each pair may become confluent by the absorption of the 

 partition between them (fig. 162). This occurs about the 

 same time that the outer wall bursts in order to set free the 

 spores. Such anthers, at the time of opening, are apparently 

 two-chambered. 



289. Dehiscence. — The opening of the chambers occurs 

 in one of three ways : by pores, by slits, or by valves. ( i ) 

 A small area of the outer wall is absorbed or breaks away so 

 that the pollen spores sift out through the pore so formed 

 (fig. 163); or (2) a crack begins at one point and extends 



