SUMMARY 429 



for consecutive argument, but their correspondence with those in 

 Lycopodium, where the argument can be more fully developed, points to a 

 clear analogy. Accordingly the facts may be held to indicate a probability 

 that here also a progressive differentiation of sterile and fertile regions 

 from an indifferent shoot which performed both functions has been 

 effective, and that abortion of sporangia has played its part. That the 

 strobilus which we see is the residual and now specialised fertile tract, 

 while the vegetative system below has been initiated, or at least extended, 

 by abortion of sporangia : this results in deferring the spore-production to 

 a later period. Appendages of two sorts are recognised throughout, viz. 

 the leaves and the sporangiophores : the former remain effective for 

 assimilation or for protection in the lower vegetative region : in the 

 strobilus the two may exist together, and even show intimate relations : 

 or the sporangiophores only may be present, as in Eyuisetitm, the true 

 leaves being absent, perhaps by abortion. 



As regards spore-output, continued apical growth and branching have 

 served as a set-off against the progressive sterilisation in the region below. 

 But in addition these plants illustrate a probable amplification of the 

 spore-producing members themselves, by septation and upgrowth of the 

 vascular placenta, resulting in the sporangiophore itself: this is a more 

 effective spore-producing member than a single sporangium. Another 

 method of advance has probably been by fission, which increases the 

 number of sporangiophores ; but this appears to have brought in its train 

 a corresponding reduction in the number of the sporangia, as seen in 

 Sphenophylhim Dawso/ii. Thus both evidences of increase and of decrease 

 in number of sporangia are illustrated in this, as in most other phyla of 

 Vascular Plants. 



Some idea of the probable origin and nature of the sporangiophore 

 has been gained by the study of the Equisetales and Sphenophyllales. 

 It is a member attaining a considerable size, and endowed with a vascular 

 system, while it carries out the important function of spore-production. 

 There is no prima facie reason why such a member should show any 

 strict limitation of size. The larger it grew, the greater would be its 

 capacity for producing fresh germs : there would thus be an inherent 

 probability of its increase, rather than the reverse. When the question 

 is asked whether such increase is illustrated in any forms of Vascular 

 Plants, a debatable ground is reached in which the Ophioglossales are 

 the subject of dispute. It is only by a careful study of their details 

 that an opinion can be formed : this will now be taken up. 



