Development of the Fern Leaf 9 



stages they appear, first as one or two here and there, then grad- 

 ually increase in number, and finally become extremely numerous. 



(2) In the mature stages of the leaf of at least one species, 

 appendages to the leaf are present which seem to be both out of 

 place and a bar to the symmetry of the leaf, but which retain 

 their positions in the preceding stages until an early stage is 

 reached in which the leaf is so altered in form that they consti- 

 tute an integral part of it. For example, in the mature stages of 

 the leaf of Adiantum pedatum a single leaflet is present below the 

 basal pinnae on one half of the dichotomous rachis. There 

 appears to be nothing about the leaf at this stage of development 

 to account for its presence, and no corresponding leaflet on the 

 other half of the dichotomous rachis; but this leaflet retains its 

 position in preceding stages until a stage is reached in which 

 the section of stalk that later forms the section of dichotomous 

 rachis bearing the leaflet constitutes the base of the rachis of a 

 pinnate branch, and the leaflet constitutes one of the basal 

 leaflets of that branch. 



Owing to lack of space, additional evidence tending to confirm 

 the truth of the above hypothesis cannot be cited here, but it is 

 believed that the reader will continually find such evidence, both 

 in facts cited elsewhere in this book and in those that come 

 under his own observation. 



The development of the fern leaf, while characterized by other 

 particulars as well in individual cases, may be said to be char- 

 acterized in general by change in the leaf's form and by increase 

 in the leaf's size and in the complexity of the leaf's venation. 

 This is true whether the leaf is simple at first and remains simple 

 throughout its development, whether it is simple at first and 



