340 REPRODUCTION OF EQUISETUM 



megaspores and microspores will be seen in the disussion 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 affects its germination and development. Perhaps this 

 physiological differentiation of the spores led to their appro- 

 priating different amounts of food during their formation in the 

 sporangium, and so they finally came to be distinguished as 

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

 archegonia present essentially the same features as were noted 

 in the Filicales, and fertilization is effected in the same manner. 

 (d) The Germination of the Gametospore. The most note- 

 worthy departure in the germination of the gametospore is seen 

 in the limited growth of the stem. The early stages are similar 

 to those of the Filicales, but after the stem has formed a few 

 nodes with three leaves each, it is replaced by a stem that devel- 

 ops at its base. This second stem attains a somewhat larger size, 

 but is finally replaced in the same manner by a third shoot. In 

 this way several stems are formed until finally one is developed 

 that penetrates the ground and forms the characteristic rhizome 

 of the mature sporophyte. These plants with their scale leaves 

 reduced to protective organs, sunken stomata and chlorenchyma 

 confined to the grooves of the internodes, present an extreme 

 form of xerophytic structure and form a sharp contrast with the 

 large and usually thin-leaved Filicales. While they occur in dry 

 localities, to which conditions their structures admirably adapt 

 them, they are of more common occurrence in moist and wet 



