106 MORPHOLOGY OF TISSUES. 



manner, as in the stem of Pieris aqiiUina, or it occurs in the form of thick-walled but 

 flexible fibres, and forming either layers and bundles (stem of Equisetum, leaf of Coniferae, 

 Fig. 8.9), or in long isolated fibres, similar to true bast-fibres (leaf of Cycadeae). In all 

 these cases the cells of the hypoderma are extended longitudinally ; but when it is re- 

 quired in addition to produce very resisting layers, the cells often extend vertically to the 

 surface of the organ, and, increasing greatly in thickness, form layers of closely arranged 

 prisms, as in the pericarp of Marsilea and Pilularia and the testa of the seeds of Papilio- 

 naceae. Isolated cells of the same kind are sometimes found in the hypoderma, as 

 accompaniments of the stomata and air-cavities {e.g. in leaves of Hakea). 



The Bundle-sheaths are commonly formed of a single layer of cells, which is in close 

 contact with and envelopes each separate fibro-vascular bundle (Fig. 83); or, when these 

 are arranged in a circle in the transverse section of the stem, forms an envelope com- 

 mon to the whole in contact only with the phloem-layers (Fig. 82). The longitudinal 

 walls of these simple bundle-sheaths placed radially always show in transverse section 

 a black point, in consequence of a peculiar folding of the wall. The walls of these 

 cells are mostly thin, but lignified or otherwise altered ; in the thinner vascular bundles 

 of Ferns on the side facing the bundle they are often much thicker and brown. In 

 many Equiseta {e. g. E. hyemale) a continuous bundle-sheath runs along the inner side 

 of the circle of the vascular bundle. In many Monocotyledons, especially Grasses and 

 Palms, each fibro-vascular bundle, the xylem and phloem of which are soft-walled and 

 deHcate, is surrounded by a layer, consisting of several strata of firm, long, lignified 

 prosenchymatous cells (Fig. 81). Much stronger layers of brown-walled sclerenchyma 

 accompany the vascular bundles in the stem of Tree-ferns. The axial fibro-vascular 

 substance of all roots is surrounded by a simple bundle-sheath generally wath thin 

 walls (Fig. 117). (On the bundle-sheaths cf. Caspary, Jahrbuch fiir wissen. Botanik. I. 

 Hydrilleen. — Sanio, Bot. Zeitung, pp. 176 et seq., 1865. — Pfitzer, Jahrbuch fiir wissen. 

 Bot. VI. p. 297.) 



The Intermediate Tissue consists of thin-walled succulent parenchyma with intercel- 

 lular spaces which are absent from all other forms of tissue ; in the stem, however, of 

 LycopodiaceaR and of many other Cryptogams the intermediate tissue consists of prosen- 

 chyma, and this is then either thin-walled as in Selaginella, or thick-walled as in Lyco- 

 podium. In so far as the intermediate tissue is parenchymatous, it may be termed simply 

 parenchyma of the fundamental tissue or Fundamental Parenchyma. Two principal 

 forms of this may be distinguished, which are nevertheless united by transitional forms, 

 1)1%. the colourless parenchyma which occurs in the interior of large succulent stems and 

 tubers and in all roots and succulent fruits, and the parenchyma rich in chlorophyll 

 which forms the superficial layers beneath the epidermal tissues of stems and fruits. In 

 the foliage-leaves, when thin and delicate, it fills up the space between the upper and 

 lower epidermis ; if they are very thick, as in species of Aloe, it forms only the super- 

 ficial layers, while the inner mass of tissue is colourless parenchyma. 



Not unfrequently there occur in the fundamental parenchyma very peculiar isolated 

 cells, groups of cells, bundles, or bands. For example, in the mesophyll of the leaves 

 of Camellia (Fig. 16, P) branched thick-walled cells appear; similarly formed spicular 

 cells occur in the parenchymatous tissues of Gymnosperms, and are especially abun- 

 dant in Welwitschia ; the polyhedral stone-cells (sclerenchyma) in the flesh of pears are 

 arranged in groups ; and a similar isolated or grouped arrangement occurs in the bark 

 of many trees ; the brown-walled prosenchymatous sclerenchyma-cells in the fundamen- 

 tal parenchyma of the stem of Tree-ferns and of Pteris aquilina appear arranged in the 

 form of bundles and bands. The sclerenchyma in the carpel of stone-fruits (the tissue 

 of the stone in Prunus, Cocos, &c.) forms closed massive layers. To this description 

 must also be referred many peculiarly thickened cells which occur here and there in 

 the parenchyma, as well as the fibrous cells of the anther-walls, if these do not rather 

 belong to the epidermal system. (Further material will be found in Schacht, Lehrbuch 

 der Anatomic und Physiologie der Gewebe, 1856 ; Thomas, Jahrbuch fiir wissen- 



