DEVELOPMENT OF PLANTS 299 



226, B) . The indusia are lacking in some forms, as in the beech 

 fern, Phegoteris, and in the polypody, Poly podium (Fig. 226, 

 C). These genera are distinguished by the fact, among others, 

 that the leaves of Polypodium drop off, leaving a scar as in our 

 deciduous trees. The sporophyll of the sensitive fern, Onoclea, 

 bears a striking outward resemblance to Botrychium, but the 

 sporangia-like bodies are really leaf bodies rolled up and each 

 bears several round sori on its inner side (Fig. 227). In this 



B 



FIG. 227. The sensitive fern, Onoclea: A, portion of normal green leaf. 

 B, a spore-bearing leaf. C, two views of one of the round lobes of B, 

 showing the sori on inner side of the lobe. After Bailey. 



and several other genera, see Osinunda, the work of photo- 

 synthesis is given over to large green leaves that do not produce 

 sporangia. 



(b) The Gametophyte. In the majority of ferns the spore 

 germinates by rupturing the outer coats and producing a germ 

 tube from which one or more delicate rhizoids are cut off. The 

 germ tube elongates, forming a short chain of cells which soon 

 develop by apical growth into a flat thalloid structure, commonly 

 called the prothallium, that is attached to the ground by numer- 

 ous rhizoids (Fig. 228). This gametophyte often becomes heart- 

 shaped (Fig. 228, C), owing to the more rapid growth of the 

 cells that are cut off from the apical cell. In some genera 

 branched filamentous or narrowly thalloid growths are developed 

 that resemble the protonema of the mosses or the thallose hepat- 

 ics. The gametophyte usually lives but a few months, although 



