THE SPORE-PRODUCING MEMBERS 693 



may have come about. In Chapter VII., which deals with sterilisation, 

 examples have been brought forward showing how widespread is the 

 conversion of individual cells, and even tracts of tissue from the fertile 

 to the sterile state, and that in some cases septation of spore-sacs has 

 actually been the result. It was concluded (p. 102) that plants show 

 not uncommonly to-day such a conversion of cells from the propagative 

 to the vegetative state as the antithetic theory would demand. Further, 

 in Chapter VIII. (p. 112) it was shown that commonly the archesporium 

 of Vascular Plants is not strictly circumscribed, but that the sporogenous 

 groups have often ragged edges : this suggests on the basis of structure 

 that each fertile tract is a residuum left by advancing sterilisation ; in 

 fact, the sporangia may in the simpler cases be regarded as islands of 

 fertile tissue which have retained their spore-producing character. In 

 Chapter XI. (p. 140), on the theory of the strobilus, it was shown how 

 the disposition of the parts in some of the simplest Pteridophytes suggests 

 as a prototype, prevalent though perhaps not general, an upright, radial, 

 strobiloid structure, consisting of a predominant axis showing continued 

 apical growth, and bearing relatively small and simple appendages formed 

 from it by enation. Associated with these are sporangia each containing 

 as its essential feature an island of fertile tissue. It is impossible to 

 bring proof how a simple strobilus such as this actually originated ; but 

 it can be claimed that all the structural and developmental facts 

 described in Part II. accord readily with a theory of origin by septation 

 from a continuous spore-sac and enation of appendages. So also is 

 physiological probability, for the sporangial types are better fitted for the 

 mechanical protection, the nutrition, and the dispersal of the numerous 

 spores than those with the non-septate sac : and in homosporous forms, 

 which all the most primitive types were, the larger the number of germs 

 the greater the probability of survival and of spread. 



Passing, however, from such hypotheses, which are not susceptible of 

 actual proof under present conditions, to matters of direct observation, a com- 

 parison of the fertile shoots of all the known homosporous Pteridophytes 

 shows them to be composed of three constituent parts : (i) the axis, which 

 the embryological comparison as well as the facts of development in 

 the growing shoot have shown to be the pre-existing part ; (ii) the bracts 

 or sporophylls, which are appendages produced by outgrowth from the pre- 

 existent axis ; and (iii) the spore-producing-members, under which general 

 term are included sporangia and sforangiophores, with their phyletic products. 

 These may be inserted either on the axis or on the sporophyll. It is 

 believed that (ii) and (iii), though they show commonly a local relation to 

 one another, have actually been distinct organs throughout descent: neither 

 has been the result of metamorphosis of the other. In further support of 

 this it will be shown that they do not bear any obligatory relation one to 

 another : either may exist without the other : while either may show fission 

 independently of the other, though in some forms both are alike in this. 



