THE MICROSPORANGIUM 221 



plants, we find a complete absence of typical transfusion tissue in 

 the leaves, although its presence has been erroneously described 

 for the group. The cycadean gymnosperms in the absence of 

 foliar transfusion tissue resemble the true ferns, which are also 

 characterized by the exclusion of tracheary tissues belonging to 

 this category from their foliar organs, whether vegetative or 

 reproductive. It is highly significant that an epidermal sporangial 

 mechanism and the absence of transfusion tissue are features which 

 alike mark the Pteridophyta and the seed plants most nearly allied 

 to these. Beginning with the Ginkgoales and proceeding upward, 

 we find transfusion tissue progressively taking the place of the 

 centripetal or cryptogamic wood in the vegetative leaves; and in 

 the reproductive leaves the transfusion tissues or structures 

 definitely associated with them assume the function of opening the 

 sporangium at the time of the ripening of the spores. The cor- 

 rectness of this interpretation of the situation is best seen in 

 Ginkgo, in which in the lower region of the sporophyll the trans- 

 fusion tissues are developed very much after the manner in which 

 they present themselves in the case of the vegetative leaves. 

 In the upper region of the sporophylls, bearing the microsporangia, 

 the transfusion elements grade imperceptibly into the reticulately 

 thickened mechanical tissues of the sporangial walls. In the sub- 

 tribe of the conifers which is beginning to assume importance as a 

 candidate for the most primitive phylogenetic position in the group 

 (namely, the Abietineae) , we find the transfusion zone not only well 

 developed in the vegetative leaves of both living and fossil repre- 

 sentatives, but likewise occurring under highly significant condi- 

 tions in relation to the sporangial mechanisms. In the remaining 

 gymnosperms the mechanical tissue shows a strong tendency to 

 become reduced in amount and loses all direct relationship to the 

 fibrovascular tissues proper. In the angiosperms, as has been 

 pointed out above, the fiber layer characteristically present in the 

 anther wall is well developed, but no longer has any relation what- 

 soever to the fibrovascular system. 



In conclusion, it may be stated that the opening mechanism of 

 the sporangia of the Pteridophyta and of the lowest gymnosperms 

 is epidermal in its origin, while that of the seed plants from Ginkgo 



