192 



A TEXT-BOOK OF BOTANY 



division of work occurs in the royal fern, climbing fern, 

 ostrich fern, sensitive fern, moonwort (Fig. 184), adder's 

 tongue, etc. 



The sporangium of an ordinary fern consists of a spore- 

 case with a slender stalk (Fig. 185). The case has a del- 

 icate wall formed of a single layer of cells; 

 and, extending around it from the stalk and 

 nearly to the stalk again, like a meridian 

 line about a globe, is a row of peculiar cells 



FIG. 185. Section through the sorus of a shield fern, showing in- 

 dusium and sporangia. After ENGLER and PRANTL. 



with thick walls, forming a heavy ring. 

 This ring is like a bent spring; and when 

 the delicate wall begins to yield, the spring 

 straightens violently, the wall is torn, and 

 FIG. 184. A moon- as ^e spring rebounds the spores are hurled 

 showing with considerable force, like a handful of 

 ^bearing pebbles thrown forward from the hand, 

 branches of a leaf. TM S discharge of spores may be seen by 



After STRAS- 



BURGER, placing some mature sporangia upon a moist 



wort, 



