330 GAMETOPHYTE OF FILICALES 



Several ferns are distinguished by false indusia that are formed 

 by the more or less modified margins of the leaf. In the bracken, 

 Pteridium, the entire membranous margin of the leaf curves over 

 the closely crowded sori (Fig. 226, A), and in the maiden-hair 

 fern, Adiantum, the sporangia are at the ends of the veins and 

 covered by reflexed portions of the leaf (Fig. 226, B). The 

 indusia are lacking in some forms, as in the beech fern, Phegopteris, 

 and in the polypody, Polypodium (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. 



Fig. 227. The sensitive fern, Onodea: A, portion of normal green leaf. 

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

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



This feature is possibly due to its more or less epiphytic habit. 

 This is the only really xerophytic fern of temperate regions. 

 The sporophyll of the sensitive fern, Onodea, bears a striking 

 outward resemblance to Botrychium, but the sporangia-like bodies 

 are really leaf lobes 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 leaves that do not produce sporangia. 



(&) 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- 



