166 



XII. STRUCTURE OF ROOTS. 



laterally by a single or double layer of parenchymatous ground- 

 tissue cells (conjunctive), and, outwards, are separated from the 

 endodermis (e) by a unilamellar pericycle (p). The endodermis 

 consists of flattened, thin- walled cells. The endodermis, the 

 pericycle, and all the other ground-tissue cells in the central 

 cylinder, are usually thickly filled with starch ; whence the bast 

 strings, which contain no starch, show up specially clearly in the 

 figure. The cells of the inner cortex are separated into unila- 

 mellar layers by numerous air-canals. In the periphery the 

 cortical cells are crowded together into a firm, strong, multi- 

 lamellar sheath. The outermost, hypodermal, cortical layer 



consists of radially elon- 

 gated cells, and here, as in 

 other roots, forms an exo- 

 dermis, which persists, 

 whilst the epidermis itself 

 dies and is destroyed. If 

 potash solution is run in, 

 the starch disappears out 

 of the cells, and the exist- 

 ence of dark shadings on 

 the radial walls of the en- 

 dodermis can be easily 

 proved. On this endoder- 

 mis, as treatment with 

 sulphuric acid shows, only 



FIG. 66. Cross-section through the root of the band which forms the 

 Acorns Calamus, m, pith; *, wood; v, bast; c V. Q/ qj rir , c i c mitinnlovico/q 

 p, pericycle ; e, endodermis ; c, cortex (x 90). snadingS IS CUtlCUlansed, 



while of the exodermis 



the whole cell-wall is. In an earlier stage, however, the exo- 

 dermis, like the endodermis, is suberised only in median strips 

 of the radial walls. The complete suberisation is connected 

 with the protective function of the layer ; the epidermis, cut off 

 by it from the living part of the root, becomes exfoliated. The 

 cells of the exodermis contain resin, By virtue of the suberised 

 band on the radial walls of the endodermis, the central cylinder 

 is rendered air-proof against the intercellular passages of the cor- 

 tex ; but in order, however, that the passage of fluids between 

 the central cylinder and the cortex may still remain possible, 

 the tangential walls are not suberised. 



Root of Iris. A cross-section through the root of Iris ftorentina 



