44 THE MORPHOLOGY OF PTERIDOPHYTES 



(in contrast to the exarch arrangement in P silo turn), (T. Vieil- 

 lardi differs in having a strand of tracheids that continues up 

 into the aerial axis in the centre of the pith region.) Whereas 

 in the rhizome there is a well marked endodermis, in the 

 aerial axes no such region can be discerned. Instead, be- 

 tween the pericycle and the lignified cortex, all that can be 

 seen is a region of cells packed with brown phlobaphene. 

 The outer cortex contains chloroplasts, but the epidermis is 

 heavily cutinized and is without stomata. These are res- 

 tricted to the leaves (and their decurrent bases) which, like 

 the stem, are also covered with a very thick cuticle, but in 

 which are abundant stomata. The leaf-trace has its origin as 

 a branch from one of the xylem strands in the stem and 

 consists of a slender strand of protoxylem and metaxylem 

 tracheids surrounded by phloem. As the stem apex is 

 approached, the number of groups of xylem tracheids is 

 gradually reduced (Fig. 6J), all the tracheids being scalari- 

 form, even to the last single tracheid. 



The vascular strand supplying the fertile appendages 

 branches into three, one to each of the abaxial leaves and 

 one to the sporangial region. The latter branches into three 

 again in the septum between the two sporangia. The early 

 stages of development closely parallel those in Psilotum,^^ 

 giving rise to thick-walled sporangia containing large num- 

 bers of cutinized spores. Both sporangia dehisce simul- 

 taneously, by means of a longitudinal split along the top of 

 each. 



When discussing the morphological nature of the fertile 

 appendages of the Psilotales, morphologists have made 

 frequent reference to abnormahties^* (the study of which is 

 referred to as 'teratology'). In both genera, the same types 

 of variation occur, some of which are represented diagram- 

 matically in Figs. 6L and 6M. The normal arrangement is 

 indicated in Fig. 6K — a lateral axis (shaded) terminating in 

 a sporangial region (black) and bearing two leaves (un- 

 shaded). In Fig. 6L one of the leaves is replaced by a com- 



