THE PTERIDOPHYTA : FILICALES, THE FERNS 



565 



however, leaf gaps are found in both steles and both 

 contribute to the leaf trace, a transverse section through 

 the stem shows two concentric rings of meristeles. 

 In Matonia pectinata there are three solenosteles, but 

 the central one, which is quite small, and may some- 

 times be protostelic, plays no part in the formation of 

 the leaf trace (Figs. 573 and 574). In Pteris podophylla 

 even four steles have been observed. 



It is worth pointing out that the historical sequence 

 of stages in the stelar evolution of the Fern is often 

 passed through during the development of the individual 

 (" Recapitulation ") (Fig. 575). Thus even the highly 

 complex polycyclic dictyostele of Marattia begins in the 

 sporeling plant as a protostele, and passes through a brief 

 solenostelic phase, firstly to a monocyclic, and finally to 

 a polycyclic dictyostele (Fig. 564, m). 



Experiments in which the leaves of various Ferns 

 were progressively removed during development of the 

 plant have shown the interesting fact that under these 

 circumstances the ontogenetic sequence is reversed, 

 the dictyostele becoming reduced to an uninterrupted 

 solenostele and then to a protostele, while in extreme 

 cases the vascular system at the apex may disappear 

 altogether. 



Finally it should be noted that dictyostely is 

 essentially difl^erent from polystely, such as is found 

 in some species of Selagine/Ia, where two or more 

 parallel steles lie side by side in the stem and are 

 unconnected with each other except at points where 

 the stem branches (see p. 580). 



nil 



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r 



J^ 



"x^ 



Fig. 575. — Gleichejiia pectinata. Longitudinal 

 plan of stelar construction showing change 

 from a true protostele in the young plant 

 through a medullated protostele to an am- 

 phiphloic solenostele in the mature plant. 

 {After McLean Thompson.) 



