124 
MORPHOLOGF OF TISSUES. 
closely packed prisms, as in the pericarp of Marsilea and Pilularia, and the testa of the 
seeds of Papilionaceae. 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. 
(ß) The Bundle-sheaths consist, in many Monocotyledons (as Palms, Grasses, 
Aroideae, &c.) ^ of elongated very thick-walled cells belonging to the fundamental tissue 
which is in close contact with each fibro -vascular bundle, either as a continuous sheath 
composed of several layers (Fig. 92, p. no), or only as a partial investment. In Ferns 
and allied Cryptogams, on the other hand, a single layer formed of peculiar cells encloses 
each separate bundle as a cylinder (Fig. 95, p. 114); and the same is the case in a few 
Phanerogams, as in the petiole of Menyanthes trifoliata, Hydrocleis Humboldti, &c. A 
layer of fundamental tissue of similar structure envelopes, as we have already seen on 
p. 115, the axial fibro-vascular cyhnder of all roots and of many stems {Lycopodium, Hydril- 
lese, Hippuris), and in most stems of Phanerogams with isolated vascular bundles (not 
forming an axial cylinder) separates the cortex from the internal tissue which encloses 
the vascular bundles and the pith. In Dicotyledons with the bundles arranged in a ring 
this layer (Fig. 93, p. 112) surrounds the mass of tissue enclosed by the cortex in such a 
manner that the separate fibro-vascular bundles are only in contact with it at their 
phloem-portion. If the axial mass of tissue of roots and stems is called plerome, then 
this layer, which separates it from the cortex, may conveniently be called the Plerome- 
sheath. In the rhizome of many Monocotyledons, as Aroideae, Zingiberaceae, Iris, 
Veratrum, it may be seen with the naked eye. The unilamellar plerome- or bundle- 
sheath consists of cells which usually become lignified at an early period, and strongly 
resist solution in sulphuric acid ; the radial side-walls and the upper and under septa are 
distinguished by a peculiar folding, which, in transverse section, gives the appearance 
of a thickening of the walls, or of a black dot. The inner walls which face the bundle, 
as well as the radial side-walls, often become greatly thickened, especially in Ferns, 
where the thickened walls frequently assume also a deep brown-red colour. 
(y) The Intermediate Tissue usually consists of thin-walled succulent parenchyma with 
intercellular spaces which are absent from all other forms of tissue ; in the stem, how- 
ever, of Lycopodiace^ and of some other Cryptogams it consists of prosenchyma, and 
this is then either thin-walled as in Selaginelleae, or thick-walled as in Lycopodieae. 
Fig. 102. — Transverse section of the acicular leaf of 
Pinns Pinaster (X about 50) ; e epidermis ; es hypodermal 
bundles of prosenchyma ; sp stomata ; h resin-passages ; 
g b colourless inner tissue enclosing two fibro-vascular 
bundles. 
Fig. 103.— The left-iiand corner of the previous figure 
(x8oo); c outer cuticularised layer of the epidermal cells; 
i inner non-cuticularised layer; c' very strongly thickened 
outer wall of the epidermal cells situated at the corner ; g i' 
hypodermal cells; .^-central lamella; 2' stratified thickening 
mass: / parenchyma containing chlorophyll; pr its contents 
contracted. 
^ Caspary, Jahrb. für wiss. Bot, vols. I and IV, p. loi et seq. — Sanio, Bot. Zeitg. 1865, p. 176 
et .s£'5'.— Pfitzer, Jahrb. für wiss. Bot. vol, IV, p. 297. — Van Tieghem, Canaux secreteurs, in Ann. 
des Sei. Nat. 1872, vol. XVI. 
