THE UNCOILING FRONDS. 19 



we have ultimate pinnules or ultimate segments for the 

 smallest divisions, though pinnule is often loosely used 

 to designate them all. 



Another peculiarity of fern fronds is the way in which 

 they are veined. Instead of giving off branches at inter- 

 vals, as in flowering plants the veins fork near the base 

 and each fork may fork and fork again. Thus one vein 

 is usually equal to any other in the frond. Commonly 

 the veins do not connect with one another, when they are 

 said to be free. If connecting they are said to be anasto- 

 mose and the meshes of the net-work thus formed are 

 termed areolce. 



Ferns are separated into families upon characters taken 

 chiefly from the rootstock, the manner of veining, and 

 the shape and position of the sori and indusia. The 

 indusium is a remarkably unvarying feature, and of itself 

 forms a kind of family escutcheon from which the genus 

 can usually be determined at a glance. Thus the indusia 

 in the Polystichums are circular; in the Woodsias, star- 

 shaped ; and in the Aspleniums, linear. The distinguish- 

 ing characteristics of the other genera may be found by 

 referring to the " key " at the back of the book. 



