THE MEGASPORANGIUM AND SEED 225 



true seed any more than that delineated in connection with the 

 last paragraph. 



An interesting condition has been described by Miss Lyon 

 in the American species of Selaginella, S. apus, and S. rupestris. 

 Here the spores germinate and are fertilized within the mega- 

 sporangia, a condition favored by the fact that the micro- 

 sporangia are situated in the upper region of the cones. The 

 microspores undergo development unshed, and on a wet day the 

 antherozoids to which they 

 give rise are able to make 

 their way to the lower 

 region of the strobilus, 

 where the germinated 

 megaspores present their 

 archegonia for fertilization. 

 The development of the 



embryo takes place after FIG. 1 66 Seedlike sporangium of M iadesmia 



the union of the sexual ele- (from Seward ' af ter Scott) ' 

 ments, and the sporelings later grow out among the leaves of the 

 cone. The situation represented by the two species mentioned 

 appears to be somewhat general for the genus and throws an 

 interesting light on the conditions which were probably present in 

 Lepidocarpon and Miadesmia. 



The most ancient types of seeds known to us have an organiza- 

 tion differing in important particulars from the seedlike structures 

 described in the two preceding paragraphs. In the first place, the 

 most antique seeds are provided with a true integument and are 

 not merely wrapped in the sporophyll as a whole. Secondly, they 

 present a very important feature in the presence of a so-called 

 "pollen chamber" which receives the microspores and provides a 

 fluid in which they may undergo germination and later effect 

 fertilization. 



A primitive type of seed is diagrammatically represented in 

 Fig. 167. The megasporangium appears within the integument 

 which covers it almost completely, so that communication with the 

 outer world is only by a narrow canal at the apex known as the 

 micropyle. The megasporangium is without any mechanical layer 



