COMPARATIVE MORPHOLOGY OF GYMNOSPERMS 157 



character of the sporangium-bearing structure, which usually 

 consists of six monadelphous stamens. Of course it is possible to 

 conceive of the so-called short axis in Ephedra and Gnetum as 

 monadelphous stamens, but this hypothesis does not seem to be 

 necessary. 



Special interest attaches to the sporangium-bearing strvicture 

 of the Cordaitales, both on account of its peculiarity, and on 

 account of the antiquity of the group. It is a debatable question 

 whether the structure is a sporophyll or a lateral axis, but in 

 any event the sporangia are erect and in a terminal cluster. 

 The terminal position of the sporangia finds its parallel in all 

 the genera of Gnetales, and is no indication of their foliar or 

 cauline character. It seems that the most primitive type of 

 microsporophyll among Gymnosperms must be that displayed 

 by Cycas and its related genera, and by the Bennettitales, but 

 the oldest microsporophylls known, those of Cordaitales, seem to 

 hold no clear relation to them. It may be that when the spore- 

 producing members of the Cycadofilices are known some light 

 will be thrown upon the evolution of the microsporophyll in 

 Gymnosperms. 



The development of the sporangium in all the forms investi- 

 gated is quite uniform, and is of the ordinary eusporangiate 

 type. The archesporium consists of a hypodermal plate of 

 cells, which divide by periclinal walls, separating an outer and 

 sterile wall layer from an inner sporogenous one. By subse- 

 quent divisions a wall of three to seven layers is developed out- 

 side of the sporogenous tissue and within the epidermis. Nu- 

 merous spore mother cells are organized, and in this condition 

 the sporangium passes the winter or the rest period. 



Gymnosperms seem to be universally anemophilous, al- 

 though it is claimed that Tumboa is probably entomophilous. 

 The extreme of anemophilous adaptation has been attained by 

 the winged pollen grains of the dominant modern genera Pinus 

 and Podocarpus. 



THE MEGASPOEANGIUM 



The presence or not of a megasporophyll is one of the prob- 

 lems of the group, which has been discussed sufficiently else- 

 where. It seems clear that the ovules are both foliar and cau- 

 line. They are certainly foliar in most if not all of the Cycad- 



