170 BIOLOGICAL LECTURES. 



devoted exclusively to the vegetative function, while others are 

 devoted exclusively to the reproductive function. In Onoclea 

 the vegetative leaf forms a large expanded triangular lamina, 

 which is divided into several large pinnae with rudimentary 

 lobes or pinnules. The reproductive leaves, or sporophylls, 

 are built on much the same general plan, but are much shorter, 

 with the pinnae and pinnules also much shorter and narrower, 

 and the edges inrolled, entirely concealing the sporangia, as if 

 they were enclosed in carpellary structures, though the sporo- 

 phyll has not become a closed structure, such as is found in 

 the ovary of higher plants. The inrolled pinnae are closely 

 appressed against the rachis of the sporophyll. 



Besides this structural dimorphism of the leaves they present 

 what is sometimes termed a seasonal dimorphism, — i.e. the 

 vegetative leaves are the first to appear in the season, from 

 April and May until July, while the sporophylls appear in July, 

 or the latter part of June. During the first part of the season 

 the vegetative or nutritive system of the plant is built up, 

 while during the latter part of the season the reproductive 

 function is in the ascendant. Occasionally an abnormal state 

 of the leaf in Onoclea sensibilis is found, in which both func- 

 tions are united in a single leaf, a portion of the leaf being 

 expanded and resembling the vegetative leaf, while some of 

 the pinnae are more or less rudimentary, revolute, and sporif- 

 erous, representing an intermediate stage between the truly 

 vegetative leaf and the typical sporophyll. This form of the 

 leaf was once described as a variety — obtiisilobata Torrey — 

 of this species of Onoclea, though later it was regarded as " a 

 rare abnormal state in which the pinnae of some of the sterile 

 fronds, becoming again pinnatifid, and more or less contracted, 

 bear some fruit dots without being much revolute or losing 

 their foliaceous character." ^ The language here suggests that 

 the sterile leaf becomes partly transformed into a fertile leaf, 

 in accordance with the old ideas of metamorphosis. More 

 recently,^ from the discovery of a number of the forms of this 



1 Gray's Manual, 5th ed., p. 668. The same form of the plant was known as 

 the species O. obUisilohata Schkuhr, idevt. 



'^ Underwood : Bull. Torr. Bot. Club, Vol. VIII ; Bot. Gaz., 1881, p. loi. 



