128 



BOTANY 



PART I 



In the Stem of a Phanerogamic plant there is an outer skin or 

 epidermis (Fig. 138 B, e) on the external surface; then follows the 

 PRIMARY CORTEX (Figs. 137, 138 A, pr), and internal to this the 

 so-called central cylinder, for which Van Tieghem has proposed 

 the name sxetj j; (column) (^'-"). The innermost layer of the primary 



endodermis 

 (Fig. 140), 

 section as 



cortex, which may be designated by the term phloeotekma, is for 

 the most part not distinctly differentiated, but can be recognised in 

 aerial stems of land-plants as a starch-sheath ; while in the rhizomes 

 of land-plants and in the stems of Avater-plant& it forms the 

 ENDODERMIS. Differentiated as a starch-sheath (Fig. 138 J, B, 

 si), the phloeoterma is rendered conspicuous by the quantity of 



movable starch contained in its 

 cells. A starch-sheath is often 

 present in the young shoots, while 

 it disappears or becomes limited to 

 certain parts of the older shoots 

 (^28), When developed as an 

 endodermis, portions of the lateral 

 walls of its cells become chemically 

 altered, as a rule suberised (^-'•'). 

 In a cross-section these suberised 

 portions of the cell walls of the 

 appear as dark .spots 

 but in a tangential 

 a wavy band. The 

 central cylinder of the stem 

 contains vascular bundles {cv), 

 which, in the Equisetaceae, the 

 Gymnosperms and Dicotyledons 

 (Fig. 138 A), are arranged in a 

 circle, whereas in Monocotyledons 

 (Fig. 137) they are irregularly distributed. In all these cases the 

 xylem portion of the vascular bundle is directed towards the centre, and 

 the phloem portion away from the centre of the stem. That part of the 

 peripheral tissue of the central cylinder lying outside of the bundles is 

 called the pericycle (pc). If the lumdlcs are arranged in a circle (Fig. 

 138 ^), that part of the central cylinder enclosed by them is the PITH 

 or MEDULLA {in) and the tissue between the different bundles the 

 primary medullary rays. In the case of scattered bundles (Fig. 

 137), a distinction between medulla and medullary rays is no longer 

 possible. Wherever there is no sharp distinction between primary 

 cortex and central cylinder, even when the tissues are young, com- 

 parative investigation alone can determine whether a tissue belongs 

 to the primary cortex or to the central cylinder. 



Altliougli tlie fuiidanieiital tissue of the iirimary cortex is niainlj^ a chloropliyll- 

 contaiuiug tissue, portions bordering on tlie ejiiderniis fretj^uently become converted, 



Fio. 137. — Transverse section of an internode of 

 the stem of i?ca Mais, pr, Primary cortex ; pc, 

 pericycle ; cv, vascular bundles ; gc, funda- 

 mental tissue of the central cylinder, (x 2.) 



