108 The Ohio Journal of Science [Vol. XVIII, No. 4, 



while that of the megasporangium is short and thick. (Figs. 

 2 and 3). The dimorphism appears very pronounced in the 

 spores. There is a remarkable difference between the micro- 

 spores and megaspores in shape and size. There is here also the 

 difference in spore development, in that the cells from the micro- 

 spore tetrad all develop while only one of the cells of the mega- 

 spore tetrad develops. There is only one spore in the megaspor- 

 angium in marked contrast to the large number in the micro- 

 sporangium. Here then is a beginning of sexual dimorphism 

 in the sporophyte. Now how is this difference in morphologic 

 expression in the sporophyte tissue -coming about? It is not 

 by any shifting of chromosomes evidently; the tissues have a 

 common vegetative origin side by side. The two kinds of 

 sporangia do not even occupy different regions of the modified 

 leaflet which bears them. It is the setting up of a differential 

 condition of the same general nature as that which determines 

 a dimorphic expression in the hard tissue of the sporocarp 

 and the membranous tissue of the leaf blade on the same 

 petiole. 



Shattuckf found both in the normal plants and in all the cul- 

 tures that a close examination revealed a homosporous tendency 

 in that microspores were sometimes formed in the megasporan- 

 gia, especially in those most distant from the nutritive supply. 

 He also found that in cases of abortion in the microsporangia 

 only one spore survives which is about sixteen times as large as 

 the normal microspore. Shattuck discovered a method by 

 which the nature of spore development in Marsilea can be con- 

 trolled to a certain degree. 



In the second stage of dimorphic advancement, as is well 

 illustrated by Selaginella kraussiana (Kunze), the strobili 

 are bisporangiate and each type of sporangium is confined to 

 a separate leaf. There are definite microsporophylls and 

 megasporophylls. In this species of Selaginella there is only 

 one megasporophyll, at the base of the cone, to about twenty 

 microsporophylls. The tissues which develop the two types 

 of sporophylls are definitely located. Dimorphism of spores 

 and sporangia is present as in Marsilea, but there are usually 

 four megaspores. When we examine the blades of the sporo- 

 phylls we also find a slight difference. They are both green and 



tShattuck, C. H. The Origin of Heterospory in Marsilea. Bot. Gaz. 49 : 

 19-40. 1910. 



