Stephenson. — Young Stages of Dicksonia and Cyathea. 13 



change is somewhat similar in Helminthostachys (Lang, 1901,. 

 " Annals of Botany "). 



Perhaps it will not be out of place to refer to the running 

 stem given off from the leaf-base in Lomaria procera. The 

 stele is at first solid, and this may grow for some distance, and 

 even branch dichotomously. But sooner or later a weak strand 

 of parenchyma cells appears in the centre of the xylem, and 

 rapidly increases in bulk. An island of sclerenchyma then ap- 

 pears in the centre of this parenchyma, and this a little later is 

 surrounded by an endodermis ; and now phloem elements are 

 clearly visible inside the xylem ring. The runner now presents 

 a robust solenostelic structure. Later, when leaves begin to 

 be given off, the leaf -gaps elongate, and typical polystely results. 



From a hurried study of Aspidium aculeatum piantlets, it 

 seemed that robust plants with a strong protostele had paren- 

 chyma cells among the xylem, and small weak plants had a small 

 solid strand. The transition is similar to Dicksonia. 



Only a study of the early stages of a large number of ferns- 

 will show whether there is any constancy in the method in which 

 the transition is made — constancy in groups of related ferns,, 

 or even in the same fern with the sporelings under varied con- 

 ditions of nutrition. I incline to think that the method of change 

 from solid strand to tubular stele is dependent somewhat oil 

 the rapidity of growth. If growth is rapid and the stem broadens- 

 quickly, some of the elements of the xylem strand will not need 

 to function as wood elements, and so will remain undifferentiated. 

 This will be the beginning of the pith. It was due in the early" 

 history of the stele to broadening of the stem, and consequent 

 loss of function of some of the more deeply placed water-carriers, 

 and these remained undifferentiated ; then the stem widened 

 further, and the segment of the xylem cut out for the leaf ex- 

 tended right to the pith ; and then phloem elements would ex- 

 tend down into the pith, because the pith, now it is not cut off 

 from the leaves by the xylem ring, can be advantageously used 

 for storage of starch. 



Polystely is only a well-marked variety of the tubular stele- 

 Here the continuous ring is broken up by gaps other than those 

 above the leaf-insertion. The change from the tube to the 

 extreme polystely of some Polypodiwms — cf. P. serpens and P. 

 nova-zelandice—is due to change of stem-habit. When the 

 rhizome becomes thick because it is used for water and starch 

 storage, and a creeping habit necessitates no mechanical strength- 

 ening, then only those wood elements of the primitive ring 

 are differentiated which are needed for water-carriage. The 

 ring could have been widened and attenuated, but this would 

 not serve so well as the network that represents the tube. 



