FILICALES 



337 



and chiefly in the apical region, numerous groups of organs which are 

 green or brown according to age. These are called son, and consist 

 of sporangia with certain protective structures. The sori vary- 

 greatly in size and form in different Ferns, which are classified accord- 

 ing to their characters. In Nephrodium, as the name implies, they 

 are kidney-shaped, as is seen in Fig. 266. Each sorus is seated on a 

 vein, which provides its necessary nourishment. It is protected by a 



Fig. 278. 



Vertical section tiirough the sorus of Nephrodium Fillx-mas. (After Kay.) 



The adaxial surface is uppermost. 



covering called the indusium, of kidney-Uke outline, beneath which are 

 numerous sporangia. If a leaf bearing mature sori be laid on a sheet 

 of paper to dry, with its lower surface downwards, the indusia shrivel, 

 and the bursting sporangia shed the spores in such numbers that they 

 give a clear print of the outline of the sporophyll upon the paper. 

 The spores are dark-coloured, very minute, and are produced in 

 millions. 



A vertical section through the sorus of Nephrodium shows an 

 enlarged receptacle, traversed by the vascular strand. The indusium 

 rising from it overarches the numerous sporangia which are attached 



K.li. V 



