XII] PLACE OF THE SORUS IN PHYLETIC MORPHOLOGY 241 



to some half a dozen or more distinct phyletic lines, are without special pro- 

 tective indusia ; but steps of abortion of them may be seen in some of these, 

 and often there is at the same time an advance to the Acrostichoid condition. 

 Frequently, however, in these Ferns hairs grow up together with the massed 

 sporangia, thus helping to protect them while young. In all of these features 

 there is material which may be used in phyletic seriation. None of them 

 can of itself be held rigidly as ground sufficient for a final decision, but all 

 must be taken into account in the general sum of characters upon which 

 such decisions should rest. 



As contributing to this end the characters of the sorus are not the most 

 trustworthy or the most important of those derived from the propagative 

 system. The sorus itself is not a constant entity; the term " monangial 

 sorus " is almost a contradiction in terms, while in the Acrostichoid state no 

 definite sori can be recognised at all. It was pointed out at the opening of 

 this Chapter that the sporangium is the essential body in spore-production. 

 It is constant in its occurrence throughout the whole series of Ferns, and no 

 life-cycle can be completed without it. It will be found that when treated 

 comparatively its characters give a consistent basis for phyletic treatment 

 far more reliable than those of the sorus which it constitutes. Neverthe- 

 less the two are intimately related biologically. In definitely soral Ferns 

 the structure of the sporangium is closely related to the constitution of the 

 sorus in which it is borne ; both take their part in the nutrition, protection, 

 and final distribution of the spores. Parts which are so mutually dependent 

 will be used in cooperation in the arguments which are to follow; and it will 

 be found that the conclusions drawn from them will yield mutual support. 



BIBLIOGRAPHY FOR CHAPTER XII 



196. Bauer & Hooker. Genera Filicum. London. 1842. 



197. BuRCK. Indusium der Varens. Haarlem. 1874. 



198. Prantl. Schizaeaceen. Leipzig. 1881. 



199. Engler & Prantl. Natiirl. Pflanzenfam. i, 4, p. 146, etc. 1902. 



200. Christ. Die Farnkrauter. Jena. 1897. 



201. Bower. Studies. Ill, IV. Phil. Trans. Vol. 1S9 (1897), 192 (1899). 



202. Bower. Studies. I— VII. Ann. of Bot. 1910— 1918. 



203. Armour. Sorus of Dipien's. New Phyt. 1907, p. 238. 



204. Davie. Peranema and Diacalpe. Ann. of Bot. xxvi, 191 2, p. 245. 



205. Schumann. Flora. 191 5, p. 201. 



206. Thompson. Deparia Moorei. Trans. Roy. Soc. Edin. 191 5. Vol. 1, p. 837. 



207. Von Goebel. Organographie. 11 Aufl. 1918. Tell ii, 2. 



208. Scott. Studies in Fossil Botany. 3rd Edn. 1920. Vol. i, pp. 250, 325. 



209. Svedelius. Einige Bemerkungen iiber Generationswechsel. Ber. d. D. Bot. Ges. 

 xxiv, 1921, p. 178. 



210. Bower. Origin of a Land Flora. 1908. Chaps, xxxii— XL. 



B. 16 



