41 8 PRIMARY ARRANGEMENT OF TISSUES. 



that both in angular and round stems and petioles longitudinal hypodermal bands 

 of fibres are found alternating with chlorophyll-parenchyma; the former project 

 toward the inside in a more or less convex form or as a strong ridge. This is the 

 case in numerous ' halms' of Cyperaceffi, species of Juncus, and Panicum turgidum. 

 In angular or channelled stems of Dicotyledons, the corresponding longitudinal 

 bands are usually collenchymatous ; they become sclerenchymatous in many Um- 

 belliferse (Chserophyllum bulbosum), Papilionacese (Spartium monospermum), and 

 the Casuarinje; further in Ephedra and the Equiseta', The same phenomena 

 appear in very wide distribution in tough hard leaves. Thick hypodermal ridges 

 of sclerenchyma, often projecting deeply inwards, traverse the lamina in Cype- 

 raceje, Typha, Sparganium, Dasylirion, Phormium, Palms, &c.^; in conjunction 

 with the vascular bundles inserted in them, to be mentioned again below, they often 

 form in flat leaves vertical plates passing through from one surface to the other. 

 Similar conditions are shown by the ribs of many tough Dicotyledonous leaves, as a 

 fibrous bundle projects into them from one or both sides, upon or between which the 

 vascular bundles are attached, e. g. Eriobotrya japonica, Theophrasta ornata, Laurus, 

 Passerina filiformis, Rosmarinus, and many others. Here also intermediate forms 

 between collenchyma and sclerenchyma frequently occur. Numerous plates of 

 sclerenchymatous fibres projecting deeply inwards on both sides traverse the leaf of 

 Welwitschia (Fig. 187, p. 408). In the leaves of the terrestrial species of Isoetes' 

 a hypodermal strand of fibres passes along the anterior and posterior surface of the 

 middle line, and in each of the two marginal angles ; between these four, additional 

 smaller ones may appear according to the species. 



In most stems and leaves the fibrous strands are sharply limited laterally 

 towards the heterogeneous bands alternating with them, quite up to the epidermis. 

 In other cases they are connected with one another by means of a continuous hypo- 

 dermal fibrous layer consisting of a few strata, into which their lateral edges are 

 Extended, and this layer follows the entire inner surface of the epidermis, and is 

 only interrupted at the stomata, e. g. stem of Equisetum hiemale, and Casuarina. 



To the latter arrangement are related the hypodermal fibrous layers, often 

 present in tough leathery leaves, which, with an average thickness of a few strata of 

 cells, run with approximate uniformity around the entire surface, becoming thicker 

 at the edges and angles, but constantly interrupted at the stomata, and rarely by 

 small gaps at other places. Such fibrous layers * occur in numerous tough leaves of 

 Bromeliacese ° (Ananassa, Bilbergia zebrina, Bromelia Caratas, &c.) ; in the leaves of 

 certain Orchids (Vanda furva, Renanthera coccinea) ; in the pinnae of many Cycadese, 

 e.g. Cycas, Encephalartos, extending sometimes all round, sometimes only on the upper 

 side ; and in most leathery leaves of Conifers " (Fig. 183, p. 380 ; Fig. 185, p. 381 ; and 



' Compare Pfitzer, in Pringsheim's Jahrb. VIII. p. 60.— Mettenius, Hymenophyllaceie, p. 439- 

 ^Jochmann, Umbelliferarum structura, p. 8. 



^ Compare Pfitzer, /. (-.— Mohl, Palm, struct. Tab. K, L.— Karsten, Veget. Org. d. Palmen. 



' A. Braun, Monatsber. d. Berlin. Acad. 1863, p. 588. 



' Cf. Pfitzer, /. r. 



' Mohl, Verm. Schriften, p. 265, Taf. X. 



" Compare Kraus, Cycadeenfiedem, /. f. — Von Mohl, Botan. Zeitg. 1871, p. 7. — Thomas, in 

 Pringsheim's Jahrb. /. c. 



