Il8 CELLULAR TISSUE. 



middle layer of the leaf of Sanseviera guineensis ; as longitudinal fibres in the 

 chlorophyll-parenchyma of the leaf of Cycas ^- They appear in an exquisite, form in 

 the parenchyma of the primary transient cortex of the root of most Coniferae (with 

 the exception of all the Abietinese); this tissue may be best placed in this category =. 

 The cells of the concentric layers of parenchyma, which lie outside the endodermal 

 sheath (§ 27), are in many forms all finely reticulated (Phyllocladus, Podocarpus sp.), 

 or thickened with coarse nets and longitudinal fibres (Cupressus spec, Sequoja 

 sempervirens) ; in Torreya nucifera this thickening is limited to the 2-3 outermost 

 layers, and the innermost layer bordering on the endodermis. In most of the in- 

 vestigated forms, as Taxus, Biota, Thuja, only the latter layer has a fibrous thicken- 

 ing,-and, as also in Torreya and Cupressus, each of its radial walls has in its middle 

 one straight, thick, stratified,. half-cylindrical, longitudinal fibre, which is continuous 

 over the transverse walls into that of the opposite radial wall, and in all cases fits 

 exactly on to a similar thickening of the neighbouring cell. In Thuja occidentalis 

 this fibre contains resin according to Reinke. The layer of cells thickened in .this 

 manner appears as a closed sheath, with the exception of Frenela rhomboidea, where, 

 according to Strasburger, it shows a break opposite both ends of the row of vessels 

 (Chapter VIII). 



As a special case, to a certain extent worthy of mention, the tabular-polyhedral 

 chlorophyll-cells peculiar to the leaves of species of Cedrus and Pinus ' and many 

 Gramina * may be further cited : these have narrowly infolded bands of wall, and 

 from them broad ridge-like thickenings of the wall protrude inwards. Compare 

 above, pp. 35, 78; figs, ii, 27. 



Luerssen ^ has recently proved that partial thickenings of the walls protruding' 

 on the outer surface are a characteristic phenomenon for the parenchyma of many 

 ferns. They occur in the chlorophyll-parenchyma of the leaf of the investigated 

 Marattiaceae, and in the parenchyma of the petiole of the same plants, as well as of 

 numerous investigated Cyatheacese, Polypodiaceae, and of Todea barbara. They 

 also occur in the stems which have been investigated with reference to this point, e. g. 

 in Ophioglossum vulgatum, species of Polypodium, and Pteris (Luerssen), Aspid. 

 filix mas, Onoclea struthiopteris, Cyathea arborea, Imrayana, Alsophylla microphylla; 

 in Marattiaceae, e. g. M. Kaulfussii, also in the cortex of the root. In most ferns 

 they appear to be wanting in the chlorophyll-parenchyma of the leaf. The pro- 

 trusions of the outer surface occur obviously only on those parts of the wall which 

 border on intercellular spaces, and, as a matter of fact, only on air-containing in- 

 tercellular spaces. In comparison with the thickness of the rest of the cell-wall they 

 are always thin, and when slightly developed they appear in the form of small knots, 

 when better perfected as fine filiform rods, rarely thickened like clubs at their ends ; 

 the longer ones are not uncommonly branched. In relatively few cases they occur 



' Compare Hofmeistel-, Pflanzenzelle, p. 168. 



^ Van Tieghem, Ann. Sci. Nat. 5 ser. XIII. p. 187.— Strasburger, Conifercn, p. 346.— Reinke, 

 Mcrpholog. Abhandl. p. 35. 



'• Meyen, Physiologie, I. Taf. VI. 17.— Hartig, Forstl. Culturpfl. Taf. 18.— Thomas, in Pring- 

 sheira's Jahrb. IV. p. 40. — Compare also Hofmeister, Pflanzenzelle, p. 169. 



* Karelstschikoff, Bullet. Soc. Imp. de Moscoii, 1868, No. i. 



" Botan. Zeitg. 1873, p. 641, Taf. VI.— Sitzungsbr. d. naturf. Ges. zu Leipzig, 187;, No. 7. 



