SCLERENCHYMA AND SCLEROTIC CELLS. 429 



shaped main bundles of the stem are immediately surrounded by a many-layered 

 zone of delicate parenchyma ; this is completely enclosed by a stout sheath, likewise 

 consisting of many layers, and reaching a thickness of above imm^ which consists of 

 acutely spindle-shaped, closely connected fibrous cells. From this sheath strands of 

 various dimensions branch off, which accompany the medullary and cortical bundles 

 (with the exception of the thinnest unsheathed branches), and those which pass out 

 into the leaf; they seldom enclose these completely as they do the main-bundles ; 

 usually they are open and runnel-shaped, and placed on the inner, medullary side of 

 the bundles, from which, however, they are separated by a broad layer of parenchyma 

 (comp. Sect. 85). In many Cyatheaceas, as Alsophila pruinata, blechnoides, and 

 species of Cibotium, all the bundles of the stem are accompanied only by open strands 

 or plates of sclerenchyma, which lie opposite their inner side. 



Connected with these are the two thick brown plates of sclerenchyma, often 

 fused to form a tube with only a narrow opening on one side, which run longitudinally 

 through the rhizome of Pteris aquilina in the middle of the parenchyma, between the 

 outer and inner tube of vascular bundles (Sect. 87). To this category belongs also 

 the axial strand of sclerenchymatous fibres, running inside the annular vascular 

 bundle, and often, it is true, bordering directly on the endodermis, in the rhizome of 

 species of Pilularia and INIarsilia. In the larger species of IVIarsilia, as M. Drum- 

 mondii, and salvatrix, the outer side of the axial annular bundle is also sur- 

 rounded by a tough brown sheath of sclerenchymatous fibres, which passes over 

 internally into thinner-walled layers of cells containing abundant starch. In 

 M. quadrifolia this sheath is replaced by a many-layered annular zone of cells 

 bordering on the endodermis, which are rich in starch, and have brown but thin 

 walls. 



Isolated small bundles of sclerenchyma with brown membranes, or even isolated 

 fibres, are observed here and there in the parenchyma of Ferns, e. g. the very hard 

 small bundles in the pith of many Cyatheacese, where, it is true, they are often con- 

 nected with those accompanying the vascular bundles as their ramifications, though 

 they may often have an independent course, e. g. Alsophila microphylla. On the 

 other hand, they are entirely absent in several species, e. g. in Alsophila pruinata, 

 blechnoides, species of Dicksonia and Cibotium, according to IMettenius. Small 

 strands, consisting of only a few fibres, ending blindly in the parenchyma above and 

 below, or isolated fibres, traverse the parenchyma longitudinally in the rhizome of 

 Pteris aquilina, Polypodium Lingua, Osmunda regalis, &c. In the winged edge of 

 the base of the petiole of Osmunda and Todea similar brown fibrous tracts and 

 fibres are arranged so as to form pinnate bands \ 



As regards the distribution of sclerotic elements in the small stem of the Lycopodia 

 and Selagitiellce, similar general rules and also similar variations prevail, as in the 

 case of the thinner stems of Ferns. In the stouter Lycopodia, as L. clavatum, alpinum 

 and Chamaecyparissus (Fig. 162, p. 349), a bulky, many-layered ring of fibres 

 surrounds the axial vascular bundle. A narrow annular zone of slightly sclerotic 

 elements lies in the middle of the thin-w'alled cortical parenchyma of the foliage- stem 

 of Psilotum. In the rhizomes of this plant and in the stems of Lycopodium Selago 



^ Compare Milde, Monograph. Generis Osmundre ; Vindob. 1S6S. 



