414 SPHENOPHYLLUM. [CH. X] 



relative disposition which is characteristic of aquatic plants: 

 the two forms of leaves may occur indiscriminately on the sam( 

 branch. The well-developed and thick xylem is not in ac- 

 cordance with the anatomical features usually associated witl 

 water-plants. It is true that in some living dicotyledons of th( 

 family Leguminosae, which inhabit swampy places, the secondarj 

 xylem is represented by a thick mass of unlignified and thin- 

 walled parenchyma, as in the genus Aeschynomene^, from whicl 

 the material of ' pith '-helmets is obtained; but the wood o\ 

 Sphenophyllum was obviously thick- walled and thoroughly 

 lignified. 



It is not improbable that the long and slender stems o] 

 this plant may have grown like small lianas in the Coal- 

 Measure forests, supporting themselves to a large extent on the 

 stouter branches of Calamites and other trees. The anatomical 

 structure of a Sphenophyllum stem would seem to be in accord 

 with the requirements of a climbing plant. It has been 

 shewn ^ that in recent climbing plants the tracheae and sieve- 

 tubes are characterised by their large diameter, a fact which 

 may be correlated with the small diameter of climbing stemj 

 and the need for rapid transport of food material. In Spheno- 

 phyllum the tracheae of the xylem have a wide bore, and ii 

 >S. insigne the phloem contains unusually wide sieve-tubes. 

 The central position of the stele is another feature which is not 

 inconsistent with a climbing habit. Schwendener and others' 

 have demonstrated that in climbing organs, as in undergroum 

 stems and roots, there is a tendency towards a centripeta 

 concentration of mechanical or strengthening tissue. The axia 

 xylem strand of Sphenophyllum would afford an efficient 

 resistance to the tension or pulling force which climbing stems 

 encounter. 



1 De Bary (84), p. 499. ^ Westermaier and Ambronn (81). 



3 Schwendener (74), p. 124. Haberlandt (96), p. 165. 



