THE PTERIDOPHYTA 



217 



real nature, more like a green and flattened stem than a leaf. These 

 facts have led many morphologists to look upon the fern-frond as a 

 structure distinct in kind from the leaf. 



In the ferns we again find the gametophyte small and inconspicu- 

 ous (Fig. 599), while the sporophyte is the generation familiarly known 

 to us. These sporophytes may be herbs, shrubs, or trees, and many of 

 them are climbers. 



Fig. 599. Organs of reproduction in the ferns: I, II, III (p), prothallium or gametopliy te ; a, tfie 

 male organ, antheridium in various stages of growth of antherozoida, which in b are shown free and 

 provided with cilia; e, oospore or egg cell: E, the archegonium — developing into young fern plant — h. 



The sporangia may be borne on the lower surface of the one form of 

 frond possessed by a species, or the sporophyll may be entirely different 

 from the other fronds. In the former case, the sporangia are grouped 

 in little masses, forming rusty- or dark-colored spots on the surface 

 which are known as Sori or Fruit-dots. These may be naked, or partly 

 or wholly covered by the reflex and modified margin of the frond, or by 

 special bract-like membranes, developed from the surface of the frond. 

 Such a membrane is called an Indusium. When the sporophyll is of 

 special form, the modes of arranging, enclosing, or protecting the 

 sporangia are various. Upon these characters, and upon those of the 

 sporangia themselves, is chiefly based the classification of the ferns. 



