CH. XIl] 



SOLITARY SPORANGIA 



207 



The sorus oi Dryopteris Filix-mas, described in Chapter I, will serve as an 

 average example for Ferns such as are neither of the most primitive nor of the 

 most advanced type. Thewhole group of organs which forms the sorus is seated 

 above a vascular strand, whose end dilates as it enters the swollen receptacle. 

 Nutrition is thus secured. Each sorus consists of numerous sporangia pro- 

 tected by an indusium. These together form an apparently coherent entity. 

 But the only constituent of it that is constant for all Ferns is the individual 

 sporangium, which ruptures so as mechanically to eject and scatter the spores. 

 The indusium is absent altogether in many Ferns. Often the sorus is not 



Fig. 197. Stauropiei-is oldha»na,'B,\m\ey. ^ = sporangium in nearly median section, attached 

 terminally to an ultimate branchlet of the rachis: j-/ = stomium. Scott Coll. 2213. 

 ^ = sporangium in tangential section attached to a short piece of the branchlet. Scott 

 Coll. 2207. C = sporangium with wall burst, attached as before: / = palisade tissue of 

 branchlet. Scott Coll. 2219. All figures x about 50. (From sketches by Mrs D. H. 

 Scott. Specimens are from Shore, Littleborough, Lanes.) 



strictly circumscribed as a definite unit. In extreme cases each sporangium 

 may be isolated, and unprotected. But in most Ferns numerous sporangia 

 are grouped together upon a single spot above a vein. Their crowded 

 grouping secures mutual protection while young, and this may be aided by 

 various accessory growths included under the general term "indusium." 

 Such a group of organs is called a ''sorus!' But its individuality is not 

 always maintained. It will be shown that in the course of Descent fusions 

 of sori, or disintegrations of them, have occurred. In several distinct phyletic 

 lines the identity of the sorus may be altogether lost, the sporangia being 

 spread uniformly over the leaf-surface. This is called the " Acrostichoid " 



