CH. XII] 



THE WATER FERNS 



499 



small adventitious dried aerial roots. These plants are 

 survivals of a type common in the first forests of the earth. 



ORDERS. HYDROPTERIDALES: THE WATER FERNS. These 

 include a few small aquatic plants, not in the least fern- 



Fig. 353. Water Ferns. 



Left, Clover-leaf Fern, Marsilia quadrifida, somewhat reduced. Above, 

 middle, sporocarps of the latter, showing megasporangia and microsporahgia, 

 X 4. Below, Salvinia natans, X i ; and above, right, one node of the latter, 

 showing the sporocarps. (From Kerner.) 



like in aspect, but bound to that group by many fea- 

 tures of their reproduction. They are further distinguished 

 by their highly developed heterospory. 



The commonest of the group is the Clover-leaf Fern 

 (Fig. 353) which grows in shallow ponds. From a creeping 

 rootstock rise slender shoots bearing at their tops two bi- 

 lobed, opposite leaves simulating four leaflets, while near the 



