218 



PTERIDOPHYTA, 



Order 1. Salviniacese. This order more nearly approaches 

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



A B 



FIG. 2l7.Salvinia -naians (natural size): A seen from above, floating on the water; 

 B a portion seen from the side in its natural position in the water. 



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

 This 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. 217 J5, 6 2 -6 3 ) ; 



the third ; the "water-leaf" (6 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 entirely closed indusium (Fig. 218). The sori are situated on 



FIG. 21S. Sori of Salvinia in longi- 

 tudinal section : h microsporangia ; m 

 macrosporangia. (x 10.) 



