142 



MORPHOLOGY 



with Lycopodiales are (i) the vascular anatomy; (2) the solitary sporan- 

 gium on the adaxial surface of the sporophyll; (3) the development of 

 the sporangium from a transverse row of 

 initial cells; (4) the trabeculae, which also 

 appear in certain fossil Lycopodiales; (5) the 

 ligule, which is present in all Lycopodiales 

 except Ly co podium and Phylloglossum ; and 

 (6) the gametophytes, which resemble closely 

 those of Selaginella. The characters not in 

 common with Lycopodiales are (i) the large 

 multiciliate sperms; (2) the absence of a 

 suspensor ; (3) the general habit ; and 

 (4) the highly specialized leaves. 



(2) PSILOTALES 



This group of pteridophytes comprises two very 

 small living genera : Psilotum (fig. 330), with two 

 species occurring in the tropics of both hemispheres ; 

 and Tmesipteris (fig. 325), with a single Australasian 

 species (sometimes more species are recognized). 

 These forms are intro- 

 duced here, not to present 

 their life histories, but to 

 illustrate very briefly cer- 

 tain stages in the evolution 

 of pteridophytes. 



Throughout Lycopo- 

 diales there appears a 

 tendency to increase the 



output of spores produced FlG $2g _ Tmesip _ 



325-328. Tmesip- by the sporophyte. The ter i s: section of sporangia 

 teris: 325, general habit, show- first and simplest method at early stage, showing 

 ing branching leafy stems was by branching, thus between the two sporog- 

 arising from a tree trunk (epi- multiplying strobili. There enous masses a sterile 

 phytic) and bearing (near tip) was ^Iso an increase in the P late of cells which is 



size of sporangia; and to form the P artition 

 , , . , , , , , that divides a sporangium 



this led, apparently for , . . 6 . . 



(single at the beginning) 

 nutritive reasons, to the into ^ sporangia . _ 



development of sterilized After B OWER- 



plates through the spo- 

 rangium, as in certain ancient lycopods ; and in Isoetes these plates almost divide 

 the sporangium into chambers. 



In Psilotum and Tmesipteris there are two further stages of development in this 



FIGS. 



the characteristic sporangia; 

 326-328, various views of the 

 paired sporangia (synangia). 

 After PRITZEL (NGLER and 

 PRANTL). 



