146 



PLANT STRUCTURES 



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 in the recoil the spores are 

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



Fig. 126.— The purple cliff brake (Pellcea a(ropurpurea), showing general habit, and 

 at a a single leaflet showing the dichotomous 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 



