146 PLANT STKUCTURES 



the stalk again, like a meridian line about a globe, is a row 

 of peculiar cells with thick walls, forming a heavy ring, 

 called the annulus. The annulus is like a bent spring, 

 and when the delicate wall becomes yielding the spring 

 straightens violently, the wall is torn, and the spores are 

 discharged with considerable force (Fig. 131). This dis- 



FIG. 126. The pm-ple cliff brake (Pelliea atropiirpnrea\ showing general habit, and 

 at a a single leaflet showing the dichotomoiis venation and the infolded margin 

 covering the sori. After MARION SATTERLEE. 



charge of fern spores may be seen by placing some sporangia 

 upon a moist slide, and under a low power watching them 

 as they dry and burst. 



Within this sporangium the archesporium (see 66) 

 consists of a single cell, which by division finally produces 



