DESCRIPTIVE FLORA 



POLYPODIACEAE. Fern Family. 

 Plants with stems (rootstocks) more or less creeping and usu- 

 ally underground, sending up leaves (fronds) singly or in groups. 

 The stem on which the frond is borne is known as the stalk. 

 Its continuation through the frond is called the rachis (plural, 

 rachises). In ours the frond is cut almost or entirely to the mid- 

 vein, never entire. When cut to the midvein the divisions are 

 called pinnae (singular, pinna) and the frond is said to be pin- 

 nate. Each pinna may be again divided, in which case the frond 

 is said to be 2-pinnate, or if cut again it is Z-pinnate, and if still 

 again, as in some Pellaeas, the frond is 4-pinnate. In this Flora 

 the ultimate division is always termed the segment. When the 

 frond is simply pinnate, as in Pellaea bridgesii, each segment is a 

 pinna. On the back of the frond are borne the fruit-masses (sori, 

 singular, sorus), usually along the veins or margins. The sorus 

 is composed of many stalked spore-cases (sporangia), each hav- 

 ing a vertical many- jointed elastic ring which, at maturity, breaks 

 transversely and somewhat straightens, thus discharging the 

 spores. The spores correspond to seeds of flowering plants. The 

 sporangia often rise from a common stalk (receptacle) to which 

 a special covering (indusium) is attached when young; some- 

 times the indusium is formed of the altered and recurved margin 

 of the frond. On germination, the spores produce flat, green 

 leaf-like tissues (prothallia) J/3 in. or less wide. These in their 

 turn produce male and female bodies that unite and grow into 

 the fern as we commonly see it. Thus all ferns pass through two 

 generations, — one asexual, the other sexual. 



frond 



Rachis 





..Stalk 



J) 

 1 



Fern Characters. — 1. A simply pinnate frond with its stalk; here each 

 pinna is a segment. 2. A 2-pinnate frond; here each pinna is parted into 

 several segments. 3. A sorus, much enlarged; I, indusium; R, receptacle; 

 S, one of the sporangia. 



