SPACES CONTAINING AIR AND WATER. 



411 



width, the air-spaces may be divided into Inkrstitial spaces, such as are of smaller 

 volume than the adjoining elements ; cavities, lacuncg, of almost equal or slightly 

 larger volume; passages, chambers, and hollows of much greater relative volume. The 

 tissue permeated by air-spaces may accordingly be termed lacunar, chambered, &c., 

 and that with only narrow interstitial spaces may be called (relatively) dense. 



The air-spaces of dense and lacunar parenchyma always arise schizogenetically 

 at the first commencement of differentiation of the tissues. In preparations seen by 

 transmitted light they may be seen close behind the growing-point, by reason of the 

 air contained in them, as black bands between the cells. Comp. e.g. in Fig. 3, p. 10, 

 the regions marked m and r. 



The interstitial spaces of dense tissue run as a rule between the rounded corners 

 of the cells, which are closely connected over the greater part of their walls, as narrow 

 angular canals, the number of their sides being equal to the number of cells which 

 border on them. Thus, e. g. the numerous three-sided prismatic interstitial air-spaces 

 in regular polyhedral cells arranged in alternating rows, the four-cornered ones between 

 the cells arranged in radial and concentric rows which are not alternate, in the inner 

 primary cortex of many roots (Fig. 51, p. 124). More rarely they form narrow slits, 

 standing serially one above another, and separated one from another by connected 

 portions of the walls, these slits lying between 

 the limiting surfaces of two contiguous cells, 

 as in the dense tissue of the leaf of Myrtacese, 

 species of Scirpus, the parenchymatous lamellae 

 of the cortex of Pilularia and Marsilia : 

 this condition is allied to that of the many- 

 armed lacunar tissue. 



Of lacunar parenchyma there may be dis- 

 tinguished two chief forms, which however 

 cannot be sharply separated. The first (Fig. 

 87) arises through the unequal growth in 

 surface of all cells as they pass from the 

 meristematic state, in such a way that at 

 certain points they put out processes which 

 may grow to long arms, at other points these 

 are not formed. The ends of the processes 

 of adjoining cells remain connected with one 

 another ; between the other parts of the sur- 

 face the intercellular space is formed. Ac- 

 cording as the processes are short, or are extended as long arms, and are arranged 

 in one or several radial planes, the form of the cells and of the cavities varies. 

 Almost all imaginable forms occur in the lacunar part of most bilateral leaves of 

 all classes, with at the same time various transitions to lamellar parenchyma. 

 " Such parenchyma composed of ' stellate ' cells occurs besides in many Monocoty- 

 ledons, especially marsh- and water-plants. As examples of this are the dia- 

 phragms of air-passages, to be mentioned below : stellate bands (which will also be 

 returned to below), alternating with the denser masses of parenchyma which contain 

 the vascular bundles, traverse the petioles of the Marantacese longitudinally (species 



Fig. 87. — Sagittaria sagittsfolia ; root, transverse section ; 

 / — ///and I — 3 two radial rows of many-armed cells from 

 the cortical parenchyma; ^arnis; ^ cavities {350). From 

 Sachs' Textbook. 



