Morphology. 27 



exquisite delicacy and the extent to which this dividing is carried 

 in some ferns gives to them their chief aesthetic value. 



The continuation of the stipe through a simple frond is called 

 the nddvein; through a compound frond is called the rachis, and 

 is further distinguished as primary when the frond is much com- 

 pounded. A frond is entire when the margin forms an unbroken 

 line ; when so cut as to form lobes extending half way or more to 

 the midvein it is caWed pinnatifid ; when these incisions extend 

 fully to the midvein the frond is said to be simply pinnate and the 

 divisions are called /z»«<^. When the pinnae are cut into lobes 

 the frond is bipinnatifid and the lobes are called segm-ents, and 

 when these extend to the secondary midveins it is bipinnate and 

 the divisions are csA&A pinnules. The secondary midvein then be- 

 comes a secondary rachis. In like manner we may have ferns 

 that are tripinnatifid and iripinnate, quadripinnatifid and quadri- 

 pinnate. The last lobes are designated ultimate segments and the 

 last complete divisions ultimate pinnules. All these various forms 

 from entire to quadripinnate are abundantly represented among 

 our native ferns. 



28. In some pinnate fronds, as in the oak-fern {Phegopteris 

 Dryopteris), the lower pair of pinnse is greatly enlarged and 

 more compound than those above, so that the stipe appears to 

 form three branches bearing similar and nearly equal portions. 

 Fronds of this character are usually triangular or pentagonal in 

 outline and this method of branching is called iernate. It will be 

 readily seen that this is merely a modified form of the ordinary 

 pinnate frond. Throughout the domain of nature there is infinite 

 variety of form and structure and at the same time, unity in plan 

 and conformity to a few generalized types of structure. 



29. Venation. — The method of veining admits of great vari- 

 ation, often serving to distinguish species and more especially the 

 sections of the various genera. In some ferns, like most shield- 

 ferns {Aspidium), the veins zx^free, that is arismg from either side 

 of the midvein they do not unite with any other vein. In some of 

 these the vein is simple, (not branched), in others variously forked. 

 In many the veins repeatedly anastomose or unite together form- 

 ing a series of network or areolee. This may be somewhat irregu- 

 lar as in Onoclea, or forming a single row of areolae next to the 

 midvein and thence free to the margin as in Woodwardia Virginica, 

 or forming many uniform areolaa by the parallel transverse vein- 

 lets connecting the distinct and parallel primary veins as in Poly- 

 podium, Phyllitidis. 



