218 



PTERIDOPHYTA. 



Order 1. Salviniaceae. This order more nearly approaches 

 the true Ferns, especially so on account of the form of the indusium. 



A B 



FIG. 217. Sah-inia natans (natural size): A seen from above, floating on the water; 

 />' a portion seen from the side in its natural position in the water. 



Only one species is found in Europe, Salvinia natans (Fig. 217). 

 Tins is a small, floating, annual, aquatic plant, entirely destitute of 

 roots. The dorsiventral, horizontal stem bears two kinds of leaves, 

 which are arranged in whorls of three. " Two of these which turn 

 upwards are oval, entire, " aerial foliage-leaves " (Fig. 21 7 7>, b 2 -b 3 ) ; 



the third, the "water-leaf" (t* 1 ) is 

 submerged and divided into a 

 number of hair-like segments, 

 similar to the submerged leaves 

 in many aquatic plants, for in- 

 stance, Water-buttercup (see also 

 Fig. 215 C). The whorls of leaves 

 alternate with each other; there 

 are thus 4 rows of dorsally-placed 

 aerial leaves, and two rows of 

 ventrally-placed submerged leaves. 

 The sporangia are situated in sori, 

 each sorus being borne on a small 

 column (receptacle or placenta) and enveloped by a cupular, 

 but <////'/Wy clnv.d indusium (Fig. 218). The sori are situated on 



I n.. -1 1\ Sori of Salriii/a in longi- 

 tudinal section : h. microsporangia; /i 



ni:irti.spMr;iiiLri:i. ( ,-. in.) 



