DEVELOPMENT OF PLANTS 



339 



the Ophioglossales than of the prothalHum of the Filicalcs. The 

 most important feature about the gametophytes is the fact that 

 they are as a rule dioecious and of two sizes, the smaller ones 

 bearing only antheridia and the larger only archegonia. This 

 difference in the nature of the gametophytes is largely due to 

 nutrition as noted among the Filicales, the well-nourished ones 

 being the female. In fact antheridia may appear upon the female 

 gametophyte as a result of insufficient nourishment during its 



Fig. 234. Fig. 235. 



Fig. 234. Sporophylls and spores of Egtiisetum: 2, sporophylls viewed 

 from outer and inner side, showing form and attachment of sporangia and 

 the central stalk attaching the sporophyll to the strobilus. 3, spores with 

 elaters expanded in a and partially coiled in b. 



Fig. 235. Female gametophyte of Eqiiisetum bearing several archegonia 

 and leaf-like lobes. At right male gamete. — After Sadebeck. 



later development. Thus we see that the germination of these 

 spores, which are apparently exactly alike, is controlled by a defi- 

 nite stimulus, just as was the case in the formation of zoospores 

 and gametes among the lower green algae (page 185). It is 

 noteworthy that some of the extinct species of this group actually 

 stored more food in certain spores than in others, so that they 

 came to differ somewhat in size. This habit is well established 

 in some of the living Filicales and will also be noted in the next 

 order. As a consequence of this tendency, the nature of the 

 gametophyte developed from the spore is no longer a matter of 

 chance. The larger spores, called megaspores, by reason of the 

 more abundant food produce female gametophytes, while the 

 smaller spores, microspores, form small gametophytes bearing 

 only antheridia. The significance of this tendency to produce 



