FOSSIL GYMNOSPERMS 



147 



by septa consisting of a single plate of cells, extend the full 

 length of the broad synangium — that is, from its base to its 

 apex, and form two rows, one on each face of the synangium, 

 probably separated from each other by a few layers of inter- 

 vening tissue. There is every evidence that the whole struc- 

 ture at first consisted of a mass of homogeneous tissue, in which 

 regularly placed sporogenous masses were differentiated, each 

 mass standing for a sporangium, exactly as in Marattia and in 

 Danaea. At maturity the tissue between the two rows of 

 sporangia apparently breaks down, at 

 least the synangium splits into two 

 valves, just as in the genera of Maratti- 

 aceae referred to. In Fig. 101 trans- 

 verse sections of the two rows of sporan- 

 gia may be seen in most cases, and in 

 a few instances a different point of view 

 has made visible the septa which sepa- 

 rate the sporangial chambers. In Fig. 

 103 this is especially evident, where a 

 single synangium presents a somewhat 

 striking resemblance to the trabeculate 

 sporangium of Isoetes. 



We have used the terms sorus and 

 synangium indiscriminately in describ- 

 ing these sporangial structures, upon the 

 hypothesis that a synangium and a sorus 

 are morphological equivalents. We 

 recognize the fact that such a synan- 

 gium as we have described may be re- 

 garded as a single plurilocular sporan- 

 gium, resulting from the sterilization of 

 potential sporogenous tissue, as in Iso- 

 etes. To discuss this question, however, 

 does not fall within the purpose of this 

 book. The important fact to the stu- 

 dent of Gymnosperms is that among 

 the Bennettitales there are forms whose microsporangiate struc- 

 tures are almost identical with those of such Ferns as Marattia 

 and Danaea, whether these structures be called sori, synangia, 

 or plurilocular sporangia. 



Fig. 102. — Cycadeoidea Daeo- 

 tensis. transverse section of 

 a. small portion of a synan- 

 gium, showing outer layer 

 of heavy-walled eells, the 

 inner more delicate tis- 

 sue, and three sporangial 

 chambers containing pollen 

 grains (p). 



