350 PRIMARY ARRANGEMENT OF TISSUES. 



separation vary in successive sections of their longitudinal course. Feeble branches 

 of higher order once more show a more radial arrangement and mode of union. In 

 the non-bilateral stems of L. Selago radial union of all the 4-6 plates is, according to 

 Hegelmaier, the more frequent case, while irregular winding and grouping are more 

 rare. For further details compare the treatises cited at p. 281. The intermediate 

 spaces between the vascular plates, which are usually smaller than the latter, are 

 occupied by the one or more masses of phloem of the bundle, each constituting a 

 correspondingly-shaped group of elongated prismatic parenchymatous cells with 

 oblique ends, and apparently oily contents, among which lies a usually simple 

 interrupted row of wider sieve-tubes, represented by the wider, somewhat more 

 strongly contoured meshes of Fig. 162 (comp. p. 181). 



The walls of all the elements of the phloem are soft, swell strongly in water, and 

 become blue with solution of iodine in potassium iodide. Between the peripheral 

 angles of the vascular plates, and alternating with them, lies in each phloem-portion 

 a small group of thick-walled, narrow, elongated fibrous elements, — the primitive 

 elements of the phloem. Round all the corners runs a zone of prismatic'parenchyma, 

 usually two cell-layers thick, of the same or similar cell- form and structure to that 

 of the phloem, but in most species (L. clavatum, annolinum) distinguished by inter- 

 cellular spaces, and loose, easily separable connection of the cells. A sheath, con- 

 sisting on the average of two layers of tangentially elongated cells, possessing thin 

 walls, cuticularised according to Russow, and not undulating, surrounds the whole 

 vascular bundle, and unites it with the inner cortex, which according to the species 

 is parenchymatous or sclerenchymatous. - 



The stout roots of Lycopodium clavatum ', Alpinum, and species of similar 

 growth, have essentially the same structure as the stems. In the two species men- 

 tioned the xylem is hexarch to dekarch, very often heptarch, and then so arranged in 

 the simplest most regular case as to form three separate plates, one being diametral, 

 while two stand symmetrically in front of the two surfaces of the first ; these two are 

 concave, with U-shaped cross-section, and with the concavity turned towards the 

 periphery. Every plate diminishes in breadth in the centripetal direction, and often 

 consists in the middle of only a single scalariform tracheide. Irregularities and inter- 

 ruptions of the plates occur similar to those in the stem. In the heptarch or octarch 

 examples of L. clavatum investigated, I almost always found one of the concave 

 plates larger, and of narrow horseshoe-like cross-section, the other smaller and much 

 flatter, with a separate, in cross-section elliptical or wedge-shaped, vascular strand 

 (in itself monarch), lying in front of its slightly concave outer surface. Other arrange- 

 ments however occur, and these are sometimes most irregular and involved. The 

 structure of the surrounding tissue and of the spaces between the vascular plates is 

 the same as in the stem. In the branches of these roots the number and arrangement 

 of the plates become reduced and simplified as the thickness diminishes ; their last 

 ramifications — and in L. Selago and inundatum all roots of every order of ramifica- 

 tion—have only a vascular group surrounded by a phloem, which is perhaps only 

 parenchymatous (?). In the branches of the root of the stouter species first-named, the 



' Nageli nnd Leitgeb, Entstehiing, &c. der Wurzeln, p. 117, &c.--Van Tieghem, Ann. Sci. Nat. 

 5 ser. torn. XIII. 



