178 



PLANT BIOLOGY 





Fig. 260. — Part of Frond of 

 Sword Fern. To the pupil: Is 



this illustration right side up ? 



or finally all around. The 

 Boston fern, Washington fern, 

 Pierson fern, and others, are 

 horticultural forms of the 

 common sword fern. In some 

 ferns (Fig. 261) an entire 

 frond becomes contracted to 

 cover the sporangia. 

 The sporangium or spore-case of a fern is a more or less 

 globular body and usually with a stalk (Fig. 257). It con- 

 tains the spores. When ripe it 

 bursts and the spores are set free. 

 In a moist, warm place the spores 

 germinate. They produce a small, 

 flat, thin, green, more or less heart- 

 shaped membrane (Fig. 262). This 

 is the prothallus. Sometimes the 

 prothallus is an inch or more 

 across, but oftener it is' less than 

 a dime in size. Although easily 

 seen, it is commonly unknown ex- 

 cept to botanists. Prothalli may 

 often be found in greenhouses where ferns are grown. 



Look on the moist stone or 

 brick walls, or on the firm soil 

 of undisturbed pots and beds ; 

 or spores may be sown in a 

 damp, warm place. 



On the under side of the 

 prothallus two kinds of organs 



are borne. These are the 

 Fig. 262. — Prothallus of a , . , . . 



fern. Enlarged. archegonium (containing egg- 



Archegonia at a ■, antheridia at b. cells) and the antheridium (con- 



Fig. 261. -Fertile and 



Sterile Fronds of the 



Sensitive Fern. 



