494 OPHIOGLOSSALES 



leaves were originally fertile. Abortion of the spike, partial or complete, 

 accounts for its occasional absence, just as in Isoetes. These two types,. 

 so similar in their embryology, are similar also in the " Selago " condition 

 seen in their stunted stocks. The one, however, bears a simultaneous 

 brush of leaves, the other, for reasons biologically intelligible, tends to the 

 monophyllous habit : this difference is only one of time, not of form or 

 of relation, and accordingly both types are equally referable to a strobiloid 

 origin, with enlargement of the leaf, and of the spore-producing part which 

 it bears. 



As regards factors of increase or decrease in number of sporangia, 

 there may be some difference of opinion according to the view taken of 

 the family as a whole. In accordance with the conclusion that the 

 spore-producing spike illustrates an upgrade of development, there would 

 be recognised as factors of increase, septation with continued apical 

 growth of the spike, its branching and occasional fission : and in the case 

 of Helminthostachys a further disintegration of sporangia and enation of 

 sporangiophores. But there is no interpolation of sporangia so common 

 a factor in Ferns. As factors of decrease there appear abortion of the 

 whole spike, abortion of sporangia at the apex, and sometimes also at 

 points lower on the spike, while a factor to be considered in addition is 

 the reduction down to one in number of leaves simultaneously expanded. 

 The factors of increase may in this case be held to have successfully 

 counterbalanced those of decrease, and the net result is a spore-output 

 that appears numerically to meet the requirements of the plants, though 

 their ultimate success in propagation is limited by the exacting conditions 

 necessary for their germination. 



