298 CONSERVATIVE ORGANS. [LECT. VII. 



puris vulgaris, the air-cells, which are on the out- 

 side of a central column, composed of vascular 

 and ligneous bundles, are divided from each other 

 by smaller cells, which are filled with aqueous 

 fluid : whilst, at every whorl of leaves, bundles 

 of vessels are given off from the central column, 

 and these are surrounded by condensed cellular 

 matter, which forms a kind of diaphragm ; by the 

 repetition of which the whole stem is divided into 

 compartments at regular intervals *. 



posed of circular or heptagonal cells, divided by septa passing 

 from the centre of each, like rays; b. the cylinder which 

 surrounds the pith ; composed of green parenchymatous mat- 

 ter, in which bundles of spiral vessels (c. d.) run, surrounded 

 by condensed cellular matter ; e. departments consisting of air- 

 cells dividing the vascular bundles ; f. bundles, apparently of 

 entire vessels, forming the striae on the surface of the stem ; 

 g. lacunae or open spaces between the pith and its enclosing 

 tube. If I might be permitted to hazard an opinion regarding 

 the use of this organization, it would be this : that, as the Rush 

 has no leaves, the green parenchymatous tube is intended to 

 perform a function analogous to that of leaves, these organs 

 consisting chiefly of a similar substance. 



* Plate 5, fig. 7, a section of a transverse slice of the stem 

 of common Mare's-tail, Hippuris vtdgaris, an aquatic plant : 

 a. the exterior, or cellular part (enclosed merely by the epi- 

 dermis), consisting of large air-cells, each surrounded by 

 smaller cells, which are generally found turgid with aqueous 

 fluid ; b. the central column, which consists chiefly of spiral 

 vessels, one of which is drawn out at c. All aquatic plants 

 contain very large air-cells ; which are most abundant in their 

 stems, if their leaves be few or comparatively small, or the 

 greater number of them is above the surface of the water ; and 



