430 



PTERIDOPHYTES (FERN PLANTS) 



The character of the sporangia and the way they are borne vary 

 much in different Ferns and are much used in the classification of 



Ferns. In the lowest group 

 of the True Ferns the spor- 

 angia are borne in syn- 

 angia, which are apparently 

 composed of united spor- 

 angia. In some Ferns the 

 sporangia are borne singly. 

 In some the sori have no 

 true indusium, but the edge 

 of the leaf folds over and 

 protects the sporangia. 

 Then in the shape of the 

 sporangia, presence or ab- 

 sence of an annulus, the 

 location of the annulus, 

 and in the numlier of 

 spores borne in a sporan- 

 gium, there are important 

 differences among Ferns. 

 Again there are two ways 

 in which sporangia begin 

 their development. In 

 some Ferns, known as 

 eusporangiates, both epi- 

 dermal and sub-epidermal 

 cells of the leaf are involved 

 in forming the sporangia, 

 while in Ferns, known as 

 leptosporangiates, the spor- 

 angia are formed entirely 

 from the epidermal cells of 

 the leaf. 



In some of the True 



Ferns the sporangia are not 



borne on ordinary leaves, 



in which case the sporo- 



phyte is differentiated into vegetative and spore-bearing regions. 



Sometimes some of the leaflets are devoted entirely to l^earing 



Fig. 381. — A sporophyte and spore- 

 producing structures of a True Fern. A, 

 a Fern sporophyte, showing roots (r), 

 stem (sO, and a leaf (0 (X about ^). B, 

 an enlarged view of the under surface of 

 a Fern leaf, bearing sori (so). C, highly 

 magnified section through a Fern leaf 

 and sorus, with section of leaf shown at 

 I, sporangia at sp, and indusium at i. D, 

 a much enlarged view of a sporangium, 

 showing annulus a and method of opening 

 to allow the spores (s) to escape. 



