SUMMARY 427 



primitive position till maturity. But in the Equisetales the pendulous 

 position is gradually assumed, the inversion of the sporangia being 

 brought about by active growth of the middle region of the sporangio- 

 phore. This inverted position was probably a derivative state, as indeed 

 its gradual appearance in the development of the individual would seem 

 to indicate. The result presumably of a similar inversion is seen in 

 Cheirostrobus and in 6 1 . Romeri, while it appears to have been general 

 in the Equisetales. 



This discussion leads naturally to the question whether in the cases 

 before us the synangial state, as seen in the Psilotaceae and in S. ma/us, 

 is the more primitive, or that with separate sporangia, as seen in Eqidsetmn 

 or in Cheirostrobus. So far as individual development can serve as a 

 guide it would point to the former, for in their first stages all sporangio- 

 phores are synangial, and the state as seen in the Psilotaceae is merely 

 the consequence of maintaining to maturity the relation of the loculi as 

 at first initiated. The condition seen in Equisetum, on the other hand, 

 is a consequence of the individual projection of each developing sporangium. 

 When this is considered together with the inversion which goes along with 

 it, it would appear probable from the story of development that the erect 

 synangial condition was relatively primitive, and the condition with separate 

 inverted sporangia a derivative state. 



In view of the initial synangial condition of all young sporangiophores, 

 the further question arises of the origin of the whole structure in descent. 

 It has been designated a placental growth : is there any clue from develop- 

 ment how it came to be? It has been pointed out (p. 414) that in the 

 young sporangiophore of Tmesipteris the origin of the tissue of the sterile 

 septum which separates the sporangia is similar to that of the sporogenous 

 tissue, while in certain reduced sporangiophores the septum may itself be 

 sporogenous. These facts point, in the simple case of Tmesipteris, to the 

 conclusion that the septum is not essentially different from fertile tissue, 

 and suggest that the whole body arose from the subdivision of a single 

 sac, together with upgrowth of the adjoining tissues. In fact, that the 

 sporangiophore is really a form of sorus, resulting from septation of a 

 primitive fertile loculus, together with upgrowth of its receptacle or 

 placenta : the separate loculi would thus be carried outwards with its 

 growth. The facts of development as well as of distribution of the sporan- 

 giophores readily coincide with this view of its probable origin. Then- 

 is a biological probability that this mode of progression to a more complex 

 state should occur, for the nourishment of separate loculi is more readily 

 carried out than that of one large one, while the scattering of the ripe 

 spores is more certain where the sporangia project. Lastly, there is 

 precedent for the conversion of sporogenous tissue into sterile in Isoetes, 

 and apparently also in Lepidodendron^ while numerous Angiosperms show 

 septa in the anther, formed by sterilisation of fertile tissue in the manner 

 here suggested. 



