Page:Journal of botany, British and foreign, Volume 9 (1871).djvu/188

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By Professor Alexander Dickson.

{Read at the Meeting of the Edinburgh' Botanical Society, April 13iA, 1871.)

The fossil remains of these plants are often so compressed that it is difficult, or even impossible, to trace the secondary spirals round the cir- cumference of the stem. In those cases, however, where there is compa- ratively little compression, — i. e. where the stem is more or less cylindrical, — the determination of the phyllotaxis is easy. Of snch stems the author had examined thirteen specimens, which may be classed according to the series of spirals to which the leaf-arrangement belongs.

A. Ordinary series, \, ^, f, |-, ^-3, etc.

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

1. Lepulodendron (Possil ironstone series) ; stem about f in.

in diameter. Secondary spirals 8 D, 13 S, 21 D. Diver- gence = \l (or possibly l\).

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

Stem about \\ inch in diameter. Secondary spirals 13 D, 21 S, 34 D. Divergence = \\.

3. Lepidudendron (Possil sandstone series) ; trunk about 2 ft.

long, with an average diameter of 20 inches. Steepest secondary spirals 55 S, 89 D. Divergence := -y^f.

b. Conjugate spirals.

4. Lepidostrobus ornatus (Bathgate coal-field) ; about f in. in

diameter. Secondary spirals 10 D, 16 S, 26 D, 42 S. Divergence = 3^^ -^ = ^-| (bijugate arrangement).

5. Knorria tax'ina (from the collection of Dr. Eankin, Car-

luke) ; slightly compressed, 4-5 in. in diameter. Se- condary spirals 15 D, 24 S. Divergence 77^ = 3*9- (t rij n gate ari'an gemen t) .

6. Lepidodendron (from Dr. Rankin's collection) ; about l|^in.

in diameter. Secondary spirals 10 D, 15 S, 25 D, 40 S. Divergence =: 13 x e'^i^ (quinquejugate arrangement).

7. Lepidodendron (Dowanhill, Glasgow, Possil sandstone

scries) ; trunk about 1 ft. long, with a diameter of 1 ft. This stem requires further study ; but 35 secondary spirals were counted to the right and 56 to the left. In all probability the secondary spirals are 7 S, 14 D, 21 S,

35 D, 5 6 S, 91 D. Divergence = ^,^xT ~ tIt ("j'ia<'t6 arrangement. So far as I am aware, no case of a 7-jugate spiral has been noted hitherto.

B. Series \, \, -f-, -i^, etc.

8. Lepidodendron (Messrs. Merry and Cunningham's Clay-

band iron pit, Carluke) ; stem 2 in. in diameter. Se- condary spirals 18 S, 29 D, 47 S. Divergence '4.\.

C. Series i, \, |, y^ etc

9. Lepidodendron (Museum, Royal Botanic Garden, Edin-

burgh) ; stem soraewliat flattened, 1-1^ in. in diameter. Secondary spirals 9 D, 1 1 S, 33 D, -37 8. Divergence

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