3 o8 REPRODUCTION OF EQUISETUM 



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

 being female; In fact antheridia may appear upon the female 

 gametophyte as a result of insufficient nourishment during its 

 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 165). 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 

 megaspores and microspores will be seen in the discussion of the 

 fourth order, but attention is called to it here because the Equise- 

 tales show very clearly how such a condition came about. It 

 would appear probable that the spores of Equisetum, although 

 exactly alike as far as we can see, must have already undergone 

 some physiological change which predisposes them to develop 

 either male or female gametophytes. If this is not the case it 

 would be difficult to explain the common occurrence of dioecious 

 gametophytes in Equisetum and the rarity of such an occurrence 

 among the Filicales. It would appear reasonable to suppose that 

 the living substance in the spore of the Filicales is not so highly 

 organized and therefore not so readily influenced by external con- 

 ditions, while the composition of the spore of Equisetum is of 

 such a nature that the amount of food placed at its disposal pro- 

 foundly effects its germination and development. Perhaps this 

 physiological differentiation of the spores lead to their appro- 

 priating different amounts of food during their formation in the 

 sporangium, and so they finally came to be distinguishable as 

 large and small spores, as noted above. The antheridia and 

 archegonia present essentially the same features as were noted 



