21 8 INTERCELLULAR SPACES. 



of the petiole of Thalia dealbata. The first are lacunar plates, consisting of many- 

 armed cells usually one layer in thickness, which have apparently resulted from 

 transverse rupture of a band of similar tissue, which originally filled the passage. 

 The others consist of a layer of relatively small-celled, dense parenchyma, which is 

 covered on each surface by a lacunar layer of many-armed cells. Similar conditions 

 of undoubtedly similar origin are found in the petioles of species of Musa. 



In the leaves and petioles of the Monocotyledons above named, and the halm- 

 internodes, which resemble them in structure (Scirpus, Juncus, Papyrus, &c.), and 

 further in the petioles of Nelumbium, the longitudinal bundles are connected by 

 more or less numerous thin transverse branches (Sects. 66, 91). These run through 

 the diaphragms, especially where the lateral walls of the air-passages are only a single 

 layer of cells in thickness, either transversely through their surface, or through their 

 margin, which abuts on the lateral wall. If the diaphragms are one layer of cells 

 thick, the vascular bundle appears as a swelling of it, several layers in thickness, which 

 may protrude either on both surfaces (e.g. Sagittaria), or only on one (e.g. Scirpus 

 lacustris). Either all the diaphragms contain a transverse bundle (e.g. Papyrus), 

 or this is the case with some of them and not with others (e.g. Pontederia, 

 Butomus) : of the two sorts in Thalia and INIusa, only the dense ones contain 

 bundles. Where all the longitudinal corners between the air-passages do not con- 

 tain longitudinal bundles, as in Papyrus, Sagittaria, and the leaf of Acorus Calamus, 

 the diaphragms with transverse bundles must be continuous transversely through 

 several passages, their arrangement is thus to a certain extent dependent upon that 

 of the vascular bundles. 



The form of the cells of lacunar diaphragms with many-armed cells, which has 

 been studied as a hobby by some ', is particularly various, and often beautiful in 

 diaphragms one layer thick, in which all the arms of the cells lie in one plane. For 

 details reference may be made to the works cited, while here some few of the chief 

 forms from stems and petioles will be noticed. 



(i) Rather regular stellate cells with long arms, having between them wide lacunae, 

 which usually correspond to the original corners of contact of three or more cells, e. g. 

 Isoetes, Villarsia, Nelumbium, Potamogeton natans, Thalia, Pandanus, Pontederia, 

 Eriophorum, Heleocharis palustris, &c. In the two latter the lacunae are bordered by 

 the arms of three cells, in outline they are rounded triangular, and contracted in the 

 middle by the swollen margins of the surfaces of contact of the pair of arms forming 

 each side : they have thus the form of a gothic trefoil, and the whole diaphragm has, 

 owing to this, and to the delicate pitting of its thick cell-membranes, a peculiar ap- 

 pearance. 



(2) Short-armed cells (a) with rounded lacunae, corresponding to the original corners 

 (e.g. leaf of Sagittaria sagittifolia, Butomus). 



{b) With a series of small round or slit-like lacunae along the original limiting surface 

 of two cells : Scirpus lacustris. 



(c) With lacunae at both places, those corresponding to the corners being larger than 

 those along the sides (e. g. Sagittaria indica, lancifolia), or both being of almost equal size 

 (e.g. IMarsilia). 



In the diaphragms of Scirpus lacustris the condition described under (r) occurs, 



1 Meyen, Phytotomie, pp. 85, 193. — Haarlemer Preisschrift, p. 138, &c. — N. Syst. d. Pflanzen- 

 physiologie, I. p. 294, &c. — Duval-Jouve, I.e. 



