DEVELOPMENT OF PLANTS 



315 



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 and several other genera, 

 see Osmunda, the work of photosynthesis is given over to large 

 green lea^•es that do not produce sporangia. 



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 views and the sori on inner side of the lobe. — After Bailey. 



{h) 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 

 in some species they may endure for years, multiplying exten- 

 sively by gemmae, and so form conspicuous mats upon the moist 

 trunks and rocks. The archegonia and antheridia are usually 

 borne upon the same gametophyte. Some genera, however, are 



