54 PAL^ONTOLOGICAL BOTANY. 



and Lepiclostrobus as arborescent Lycopodiacea) having their 

 affinities with Ly cop odium, as they have all microspores only 

 in their fructification. 



The scales upon the Lepidodendron stems, as well as those 

 in the cones, are arranged in a spiral manner, in the same 

 way as plants of the present day. Professor Alexander 

 Dickson has examined the phyllotaxis of Lepidodendrons, and 

 gives the following results of his observations (Trans. Bot. 

 Soc. Edin. xi. 145). The fossil remains of Lepidodendrons 

 are often so compressed that it is difficult, or even impossible, 

 to trace the secondary spirals round the circumference of the 

 stem. In those cases, however, where there is comparatively 

 little compression, i.e. where the stem is more or less cylin- 

 drical, the determination of the phyllotaxis is easy. Of such 

 stems he has examined fifteen specimens, which may be 

 classed according to the series of spirals to which the leaf- 

 arrangement belongs : — 



A. Ordinary series, i i, f , f, -^^ etc. 

 (a.) Single spirals (D turning to the right, S to the left). 

 (1.) Lepidodendron (Possil Ironstone series). Stem about |- 



of an inch in diameter. Secondary spirals 8 D, 1 3 S, 2 1 D. 



Divergence = ^|- (or possibly -f^). 

 (2.) Lepidodendron (Knightswood, near Glasgow, Mr. J. Young). 



Stem about IJ inch in diameter. Secondary spirals 13 D, 



21 S, 34 D. Divergence = fi. 

 (3.) Lepidodendron (Possil Sandstone series). Trunk about 



2 feet long, with an average diameter of 20 inches. Steepest 



secondary spirals 55 S. 89 D. Divergence == y^^j^^^. 

 (b.) Conjugate spirals.* 

 (4.) Lepidostrohus ornatus (Bathgate coal-field). About | of 



an inch in diameter. Secondary spirals 10 D, 16 S, 26 D, 



42 S. Divergence = ~~^ (Bijugate arrangement). 



* Conjugate spirals result from u-horls of usually 2, 3, 5, 8, etc., 

 leaves arranged so as to give 2, 3, 5, 8, etc., parallel spirals, each 

 with an angular divergence equal to ^, ^, ~, ^, etc., of one of the 

 fractions expressing the divergence in an arrangement of alternate 

 leaves. 



