174 BIOLOGICAL LECTURES. 



The venation also is very prominent, and strong in contrast 

 with that of the sterile leaf, especially the lateral veinlets of 

 the pinnules. This character is quite persistent, and enables 

 one to differentiate the leaves frequently when no indusia are 

 present, and the pinnules are not obtuse, or no more so than 

 those of the normal sterile leaves. 



One transformed sporophyll had a length of twenty-five cm., 

 and a spread of pinn^ twenty-two cm. in extent. A very few 

 of the basal pinnules had their margins slightly revolute, and 

 quite a number of indusia were present, while very few of the 

 pinnules were obtuse. The venation of the entire leaf was 

 quite coarse and very prominent. 



During the expansion of the pinnules, the position of the 

 indusia changes somewhat, and its form, to a very great extent. 

 It becomes located at a greater distance from the base of the 

 lateral veinlets, the distance depending to some extent upon 

 the extent of the transformation of the pinnule. In broadly 

 expanded pinnules at the termination of the pinnae, the indusia 

 are frequently moved on to veinlets of the second and third 

 order, instead of those of the first order. 



While indusia are present on partially or nearly expanded 

 pinnules, or frequently on fully expanded ones, sporangia 

 appear to occur only on those which are entirely normal, or 

 only partially expanded. Passing through the various transi- 

 tions of the pinnules, from the normal fertile ones to those 

 which are little more than half expanded, the sporangia vary, 

 gradually decreasing in number and perfection of development, 

 as the pinnules partake more and more of the vegetative char- 

 acter. In the partially expanded pinnules, the indusium is fre- 

 quently so small that it affords very little protection to the 

 sporangia, and the location of rudimentary sporangia is often 

 at a considerable distance from the indusium. 



When the leaf has lost so much of its reproductive function 

 that the sporangia are becoming rare or rudimentary in the 

 sorus, apospory frequently occurs, and the placenta develops 

 among the rudimentary sporangia prothalloid growths. These 

 are filamentous, lanceolate, or spathulate in form, the two latter 

 forms being two or more cells wide. The cells are richly pro- 



