134 



MORPHOLOGV OF TISSUES. 



closely packed prisms, as in the pericarp of Marsilea and Pilularia, and the testa of the 

 seeds of Papilionaceac. Isolated cells of the same kind are sometimes found in the 

 hypoderma, as accompaniments of the stomata and air-cavities, e.g. in leaves oi Hakea. 



0) The Bundle-sheaths consist, in many Monocotyledons (as Palms, Grasses, 

 Aroideae, &c.) S 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 Humboldtiy &c. A 

 layer of fundamental tissue of similar structure envelopes, as we have already seen on 

 p. 1 15, the axial fibro-vascular cylinder of all roots and of many stems {Lycopodium, Hydril- 

 leae, 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 



O--^ 



Fig. 102.— Transverse section of tlie acicular leaf of 

 Pinns Pinaster {X about 50) ; e epidermis ; es hypodennal 

 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 ; g central lamella ; if stratified thickening 

 mass : / parenchyma containing chlorophyll ; pr its contents 

 contracted. 



this layer, which separates it from the cortex, may conveniently be called the Plerome- 

 sheath. In the rhizome of many Monocotyledons, as Aroideae, Zingiberaceae, /r/V, 

 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 Lycopodiaceae 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. 



^ Caspary, Jahrb. fiir wiss. Bot, vols. I and IV, p. loi et seq. — Sanio, Bot. Zeitg. 1865, p, 176 

 et seq. — Pfitzer, Jahrb. fiir wiss. Bot. vol. IV. p. 297. — Van Tieghem, Canaiix secreteurs, in Ann. 

 des Sci. Nat. 1872, vol. XVI. 



