12 BIOLOGICAL LECTURES. 



the extraordinary development of the vegetative organs of the 

 sporophyte, but must confine our attention to the evolution of 

 the spore-bearing organs, or sporangia. 



The gradual development of the sporangium is well illus- 

 trated in the true ferns, or Filicinese, the predominant type, 

 at present, of the Pteridophytes. 



We have already referred to the beginning of a segregation 

 of the sporogenous tissue in Anthoceros. The nearest ap- 

 proach to this condition in the Pteridophytes is found in the 

 peculiar genus Ophioglossum (Fig. 5, a). Here the spore- 

 bearing part of the leaf has the form of a spike with two rows 

 of sporangia which are hardly indicated at the surface, but 

 consist practically of large cavities filled with spores sunk 

 below the surface of the leaf and opening at the surface by 

 a transverse slit. It has even been claimed that here, as in 

 Anthoceros, there is a continuous layer of potential sporoge- 

 nous tissue which becomes divided into separate sporangia by 

 the sterilization of portions situated at regular intervals. A 

 very important difference between the sporangia of Ophioglos- 

 sum and the sporogenous tissue of Anthoceros is that in the 

 latter all the spores are discharged through a single opening at 

 the apex of the sporophyte, while in Ophioglossum each group 

 of spores has its own opening at the surface of the leaf. 



Starting with the very simple type of sporangium found in 

 Ophioglossum, we find a very perfect series of forms still 

 existing, which lead up to the type found in the more special- 

 ized ferns. Thus within the genus Botrychium (Fig. 5, c, d), 

 which is closely allied to Ophioglossum, there are several 

 species which illustrate most beautifully almost every interme- 

 diate form. The simplest ones have a few large sporangia, 

 more or less sunken in the tissue of the leaf, and closely 

 ■ resembling those of Ophioglossum ; while the highest mem- 

 bers of the series have numerous small sporangia which are 

 borne upon a short stalk or pedicel, and are very much like 

 those of the ordinary or leptosporangiate ferns, whose sporan- 

 gia are developed entirely from the epidermal tissue. The 

 genus Osmunda, which includes the common cinnamon fern of 

 the eastern United States, is in certain respects intermediate 



