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APPENDIX 
sporangia are formed everywhere on the veins, but not on 
the green lamina between the veins; the fructification is 
in this case called Grammitoid. In most ferns, the 
sporangia are gathered into definitely formed and placed 
clusters called sori. These are born on the dorsal or 
under surface of the leaf, but are called marginal if 
produced at or very close to the margin, and dorsal if at 
some distance from the margin. The form of sorus is 
very important in classifying the ferns of the great family, 
Polypodiaceae. The sorus may be without special cover¬ 
ing, in which case, it is said to be naked or “ exindusiate; 
or it may be provided with a covering either by the 
folding back of the margin over marginal sori, or by a 
special outgrowth of the leaf called the indusium. The 
indusium may be fastened by the middle, when it is said 
to be peltate, or it may be fastened by a point in the 
margin, or by a part or the whole of the margin. 
