PARENCHYMA. SCLEROTIC CELLS. ENDODERMIS. 



131 



are equally thickened all round, or less thickened on one side, are highly ' lignified '; 

 in some few cases they are quite or almost colourless (e. g. stem of Lycopodium) 

 usually they are coloured a dark brown. On the 

 chemical property of the characteristic brown substance 

 nothing certain is known. The sclerotic-tissue elements 

 are generally of elongated prismatic form, either with 

 slightly inclined or sharp-pointed ends, in the latter 

 case they are fibrous cells or fibres. According to the 

 character of their contents, they must, in compliance 

 .with the fundamental ideas above laid down, be for the 

 most part assigned to the category of cellular tissue, 

 since most of the elements, even those with yery thick 

 walls belonging to the dark brown layers and strands 

 in the ferns, are densely filled with starch grains, which 

 (as was observed in rhizomes of Osmunda regalis) 

 gradually disappear as their age increases. It was not 

 possible by any means to prove that these cells are 

 capable of division. On the other hand, there occur 

 side by side with these sclerotic cells, and often con- 

 nected with them by quite gradual transitions, elements 

 thickened till the cell cavity almost disappears, and 

 showing only the last traces of cell-contents. These, 

 regarded independently, should be accounted as specific sclerenchymatous fibres; 

 thus, e. g. in the brown sclerenchymatous sheath of the stem of Marsilia salvatrix. 



Sect. 27. The name Endodermis, proposed by Oudemans^ for a special case, 

 here denotes generally those peculiar limiting layers to which Caspary ^ has given 

 the name protective-sheath (Schutzscheide) '. They belong to the category of cellular 

 tissue by reason of the nature of their contents, and their power of independent 

 growth and division, which is so often to be observed, e. g. in roots of Dicotyledons. 



The endodermis is a sheath consisting in all cases of one single layer of cells. 

 It should also be observed here, that it lies as a rule at the limit between masses of 

 parenchyma and other systems of tissue, especially vascular bundles, and is then to 

 be recognised both by its development and its mature properties, as the layer of the 

 parenchymatous mass bordering on the unlike part. In roots with an axile vascular 

 cylinder the latter is always enclosed by it. The same is the case in stems with an 

 axile vascular cylinder, as Hippuris, Callitriche, Ceratophylluna, Utricularia, Elodea, 

 species of Potamogeton, Corallorrhiza, &c. (compare Chapter VIII), or with a closely 

 compressed axile system of bundles (species of Potamogeton, Hydrocotyle vulgaris, 

 &c.) : also in stems of Phanerogams with a strongly developed cylinder containing the 

 vascular bundles, this is marked off from the surrounding mass of parenchyma by 

 a layer of endodermis, e. g. Tagetes patula, and other Compositae ', Cobsea scandens, 



FIG. 48.— Two sclerotic brown cells from 

 the hypoderraal layer of the rhizome of 

 Pteris aquilina, isolated by chlorate of 

 potash and nitric acid. A more strongly 

 thicltened on one side and with branched 

 pits (530) ; S less thick-walled j the optical 

 section of the wall and the pitted wall at 

 the back are drawn in. From Sachs' Text- 

 book. 



* Ueber den Sitz der Epidermis bei den Luftwurzeln der Orchideen. Abhandl. d. Acad. Am- 

 sterdam. Math. phys. Klasse IX (1861). 



' Pringsheim's Jahrb. I. p. 441 ; ibid. IV. p. loi. 



' [Cf. also Schwendener, Die Schutzscheide u. ihre Verstarkungen, Abhandl. d. Konigl. Akad. 

 d. Wiss. zu Berlin, 1882'.] * Van Tieghem, Ann. Sci. Nat. torn. XVI. p. 113. 



