SPACES CONTAINING AIR AND WATER. 213 



If in the stellate lacunar tissue one arm of a cell be separated by a septum 

 as a special cell from the central portion, as often happens, e. g. in Jussiaea, and 

 if this cell divides further, the intercellular space is no longer limited on all sides by 

 parts of branched cells, but, according to the number and direction of the successive 

 divisions, by whole cells or by rows of cells, or simple or compound layers, or 

 lamellcE. We may distinguish the more developed forms of this arrangement from 

 the stellate tissue as lamellar porous parenchyma, though it is obvious that f'-cquent 

 intermediate forms prevent a general sharp distinction. 



In the formation of lamellar parenchyma, unequal surface-grow^th and divisions 

 of the cells which are passing from the meristematic stage keep pace with one 

 another, at least at first, so that the widih of the air-cavity and the number of cells 

 which limit it laterally increase simultaneously : it is only in the last stage that the 

 cavity is increased by extension of the cells alone \ It is not certainly proved 

 whether the other case, which may be observed in the formation of many secretory 

 reservoirs, and in the secondary cortex of Dicotyledons, viz. that the intercellular air- 

 space appears after the divisions in its vicinity are ended, occurs also in the primary 

 parenchyma. 



Lamellar parenchyma occurs in certain plants in the same positions as stellate 

 parenchyma in others ; e. g. lacunar layer of the bifacial leaves of Ilex aquifolium, 

 Arbutus Unedo, Eugenia australis, Camellia : base of the petiole of Aspidium filix 

 mas and its allies : cortex of the rhizome of Carex disticha : stems of many Aroidese, 

 as Acorus Calamus, Calla, IMonsterinese, &c. : pith of Saururese, 



As a special intermediate phenomenon between lamellar and stellate parenchyma 

 may here be named the layer consisting of loose and irregularly connected rows of 

 cells, which lies in the Selaginellas between the firm bundle sheath and the tough and 

 dense surrounding tissue. (Comp. Fig. 131, Sect. 78.) 



Wide air-containing chambers and canals, the diameter of which greatly exceeds 

 that of the surrounding cells, are produced in two ways. Those of the one category 

 arise schizogenetically, and are only distinguished from the lacunae of lamellar 

 parenchyma by their width. The others are formed lysigenetically, or better rhexi- 

 genetically : during their development a mass of tissue lying in the direction of the 

 subsequent cavity ceases to follow the surface-growth of that surrounding it, and, since 

 growth continues in the latter, the former is ruptured and more or less destroyed. 



To the first, schizogenetic, category belong the larger air-spaces in stem, roots, 

 and leaves of many marsh and water-plants ; Marsiliaceae, Salviniaceae, leaves of 

 Isoetes, Ceratopteris ; Potamogeton, Hydrocharidese, Alismacese, Pontederia, Aroideae, 

 Lemna ; Papyrus (?) ; Ceratophyllum, INIyriophyllum, Hippuris, Trapa, Hottonia, 

 Elatine, Utricularia, Menyanthea?, Nymphseaceae, Nelumbium, pith of Desmanthus 

 natans, &c. Comp. Figs. 88, 112, 122, and 124, Chap. VIII. 



To the category of lysigenetic forms belong the air-passages of the Equiseta, 

 those of the leaves, stems, and roots of most Cyperaceae, and Gramina, the leaves of 

 Sparganium, Typha, Iris pseudacorus, and its allies, Pandanus, the Marantaceae, 

 Musa (?); in part those of the stem of Callitriche, those of the leaves of the narrow- 

 leaved Eryngia, of Lobelia Dortmanna, Nelumbium (?), &c. ; lastly, the axile tubes of 



* Compare, e. g. Frank, /. c. — Vochting, Myriophyllum, N. acta Acad. Leop. vol. 36. 



