Development of the Fern Leaf 1 1 



the species in question. The remarkable part of it is, not that we 

 should find, as we do, some steps missing in the case of some 

 species, but that we should find, as we do, so few steps missing 

 in the case of many species. Often the gradations of change 

 from a simple leaf or leaflet to a compound one are so minutely 

 portrayed that the series can be likened to a series of photo- 

 graphs, taken by a kinetograph, of a single leaf undergoing 

 actual enlargement and subdivision. 



For a simple leaf or leaflet to become compound, and for a 

 compound leaf or leaflet to become more compound, one or 

 more incisions, according to the number of segments* to be 

 formed, must occur in the leaf or leaflet. Often the appearance 

 of such an incision is first indicated by a shallow sinus or small 

 notch in the leaf's margin. This sinus or notch then deepens 

 and so forms the incision. 



If enlargement of the segments takes place (and it usually 

 does), it may take place during, before, or after formation of the 

 incision or incisions by means of which the segments are formed. 

 Sometimes the portion of leaf that is to become a segment 

 elongates in the direction its apex is to take before any incision 

 that is to separate it from the remainder of the leaf is even indi- 

 cated. In such a case, a more or less extended lobe is formed. 

 We have examples of such lobes in some of those often seen 

 at the bases of the divisions of the leaf-blade in Pellaa atropur- 

 purea. In the latter plant the incisions setting free the lobes 



* The term "segment" is used here and generally throughout this book, not in its 

 restricted sense, in which it means "one of the lobes of a pinnatifid leaf or leaflet," but 

 interchangeably with the term "leaflet," as meaning "one of the subdivisions into which 

 a leaf or leaflet naturally divides." For example, the pinna of a pinnate leaf is a "seg- 

 ment" or "leaflet" of this leaf. 



