54 THE MORPHOLOGY OF PTERIDOPHYTES 



A Urostachya 



1 Selago 



2 Phlegmaria 



B Rhopalostachya 



1 Inundata 



2 Clavata 



3 Cernua 



Members of the Urostachya never have creeping axes, but 

 have erect or pendulous dichotomous aerial axes, according 

 to whether they are terrestrial or epiphytic. Their roots 

 emerge only at the base of the axes, for although they have 

 their origin in more distal regions, they remain within the 

 cortex (many being visible in any one transverse section of 

 the stem). Perhaps the most important character, phylo- 

 genetically, is the lack of specialization of the sporophylls 

 which, as a result, resemble the sterile leaves more or 

 less closely. Another characteristic is that vegetative repro- 

 duction may frequently take place by means of bulbils. 

 These are small lateral leafy stem-structures which occur in 

 place of a leaf and which, on becoming detached, may 

 develop into complete new plants. The members of the 

 Rhopalostachya, by contrast, never reproduce by means of 

 bulbils. They are all terrestrial and, although the first formed 

 horizontal axes may be dichotomous, those formed later 

 have the appearance of being monopodial, by reason of their 

 unequal dichotomy, as also do the erect branch-systems. 

 Roots may emerge from the leafy branches, particularly in 

 the creeping parts of the plant. 



Of the two sections, the Urostachya (and in particular 

 those belonging to the Selago subsection) are usually re- 

 garded as the more primitive. The British species Lycopodium 

 selago is illustrated in Fig. 9A. Its sporophylls (Fig. 9B) are 

 very similar indeed to the sterile leaves (Fig. 9C) and occur 

 at intervals up the stem, fertile zones alternating with sterile. 

 L. squarrosum shows a sUght advance on this, in that the 



