142 



MORPHOLOGY 



with Lycopodiales arc (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 Lycopodium and Pliylloglossum; and 

 (6) the gametophytes, which resemble closely 

 those of Selaginella. The characters not in 

 common with Lycopodiales are (1) 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 32Q _ Tmesip _ 

 by the sporophyte. The t cris: section of sporangia 

 first and simplest method at early stage, showing 

 was by branching, thus between the two sporog- 

 multiplying strobili. There enous masses a sterile 

 was also an increase in the P late of cells which is 

 size of sporangia; and to form the V^liUm 

 this led, apparently for that divides a sporangium 

 . . , (single at the beginning) 



nutritive reasons, to the . \ t 



' into two sporangia. — 



development of sterilized Mtei BowER . 



plates through the spo- 

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

 the sporangium into chambers. 



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



Figs. 325-328. — Tmesip- 

 teris: 325, general habit, show- 

 ing branching leafy stems 

 arising from a tree trunk (epi- 

 phytic) and bearing (near tip) 

 the characteristic sporangia; 

 326-328, various views of the 

 paired sporangia (synangia). 

 — After Pritzel (Engler and 

 Prantl). 



