412 



SPHENOPHYLLALES. B. PSILOTACEAE 



the fertile shoot is very lax, and does not form a definite strobilus, while 

 foliage leaves are interspersed irregularly between the sporophylls. There 



is thus a " Selago " condition in 

 Psilotum also, and it is even more 

 obvious than it is in Tmesipteris. 

 Psilotum is also open to deviations'" 

 of structure of the spore-producing 

 parts : reduction of the sporangia 

 from the normal three to two is 

 not uncommon, though it appears 

 frequently to be the result of arrest 

 of one of the loculi. In other 

 cases the number of the loculi 

 may be increased to four or five. 1 

 Thus in the Psilotaceae sporangio- 

 phores may be found bearing 

 sporangia variable in number from 

 one to five. The observations of 

 Thomas also show 2 that there are 

 fairly numerous instances in Psilo- 

 tum of a second dichotomy of one, 

 or even of both branches of the 

 forked sporophyll : in the former 

 case two sporangiophores are 

 present and three minute leaf-lobes, 

 in the latter case three sporangio- 

 phores and four leaf-lobes. The 

 synangia are in these instances 

 closely crowded together, and in 

 some cases at least irregular quin- 

 quilocular synangia are due to the 

 fusion of two original primordia in 

 close proximity. There are no 

 statements as to the position in 

 the fertile region which these 

 abnormalities hold. From the 

 above facts in Psilotum, as well 



aS th S6 in TmesiptertS, Thomas 



Hrnwc trip> rnnrlncinn " f haf rpnparpr? 

 araWS tne ' 



dichotomy of the Sporophylls of 



' 



the family Psilotaceae is an ancient 

 feature." Without accepting this position, this much at least is clear, 

 that the present Psilotaceae possess morphological possibilities of further 



1 Solms Laubach, Ann. fard. Bot., Buitenzorg, iv., p. 174. 

 '* Proc. Roy. Soc., vol. Ixix., p. 349. 



FIG. 229. 



Psilotum triquetn<m, Sw. Shoot showing repeated 

 dichotomy, bearing minute tooth-like sterile leaves, and 

 turgid synangia attached to the minute forked sporo- 

 phylls. It is to be noted that a sterile region intervenes 

 between two fertile regions. Natural size. 



