SCLERENCHYMA AND SCLEROTIC CELLS. 4^7 



rhizome of Pteris aquilina and Polybotrya Meyeriana. The sclerosis which occurs 

 at an early period in the cortex of the root of many Ferns was mentioned above 

 at p. 413. 



In the petioles and ribs of the leaves in Ferns, it is a general rule that a more or 

 less strongly developed sclerotic hypodermal layer, often interrupted by the bands 

 and islets covered with an epidermis without stomata mentioned at p. 405, lies 

 directly beneath the epidermis, which itself not unfrequently takes part in the 

 sclerosis. The coUenchymatous zone in the petiole of the Marattiaceae is more 

 deeply seated, and separated from the epidermis by several layers of thin-walled 

 parenchyma (comp. p. 120). \ 



According to Mettenius, strands usually branch off from the hypodermal 

 sclerenchyma in the petioles and ribs, which accompany the finer ramifications of the 

 vascular bundles into the lamina of the leaf. 



Among strictly hypodermal sclerenchymatous masses in the lamina of the leaf, 

 the continuous layer of Acropteris australis has already been described at p. 132. 

 Such masses occur in the form of nerve-like bands in many leaves of Marsilise 

 and Ferns. 



In the lamina of the aerial leaves of Marsilia coromandeliana, trichopoda, muscoides, 

 and distorts ', narrow colourless bands of sclerenchyma run between and in a similar 

 direction to the nerves ; some of these are small strands or even isolated fibres, adjacent to 

 the epidermis of the lower surface of the leaf ; others are thicker, extending through the 

 entire thickness of the leaf, from one epidermal surface to the other. Hypodermal 

 strands distributed like nerves are described by Mettenius as occurring in the segments 

 of the lamina of the leaf of Todea Hymenophylloides, Polypodium solidum, Pteris 

 pinnata, Davallia elata and elegans. In other Ferns the edge of the leaf or of its 

 segments is entirely or partially rimmed by a hypodermal (many-layered) strand 

 of sclerenchyma, which is prolonged continuously into the sclerenchyma of. the 

 petiole. This is the case in Polypodium Lingua, sporadocarpum, Brownianum, Asple- 

 nium lucidum, Polybotrya cervina, Meyeriana, Aspidium falcatum, Adiantura denti- 

 culatum, &c.^ The strands described by Mettenius as Nervi spurii, in the leaves of 

 many species of Trichomanes, may also be mentioned here, although, as running 

 through a usually single-layered lamina, they do not strictly belong to this category. 

 They consist of one or a few rows of elongated elements, which are usually accom- 

 panied by stegmata (p. 128). 



Around the vascular bundles in the stem, roots, petioles, and the stouter ribs 

 of the leaf, sclerenchyma and sclerotic cellular tissue is in many cases entirely absent, 

 the bundle or its endodermis is surrounded by thin-walled parenchyma, which differs 

 scarcely if at all from that lying further away from it, and is never sharply marked 

 oif. This is no doubt the case in most roots ; in the stem and petiole of the 

 Marattiae, in the rhizomes of Aspidium Filix mas, Onoclea Struthiopteris, Polypodium 

 vulgare, Davallia pyxidata, &c. Pteris aquilina may also be mentioned here. In 

 several roots on the other hand, and in most stems and petioles of Ferns, distinct 

 sclerotic sheaths and strands occur in company with the vascular bundles. This 

 is the case, for example, in the roots of many Polypodia, as P. ireoides, vulgare, &c., 



' A. Braun, Monatsber. der Berlin. Acad. 1870, p. 693. 

 * Mettenius, Hymenophyllacese, p. 438. . 



