32.] 



CHAPTER II. THE TISSUES. 



167 



connected with the pericycle by interfascicular conjunctive tissue 

 constituting the medullary rays. Pith and medullary rays are 

 generally absent from the steles of a polystelic member. 



Structure. The Pericycle (at one time generally termed the 

 pericambium) is altogether wanting in a few cases only; it is 

 absent when the endodermis consists of two layers (see p. 165) ; it 

 is also absent in the roots of Podostemaceae, rootlets of Pontederia 

 crassipes, in the stem of Ceratophyllum and probably of other 

 water-plants. 



It is usually a continuous membrane ; but in some cases it is 

 interrupted by projections of the vascular tissue (e.g. by the 

 xylem-b undies in the root 

 of some Gramineae and Cy- 

 peraceae ; by the phloem- 

 bundles in the roots of 

 Potamogeton, Naias, Zos- 

 tera). It may consist 

 throughout of a single 

 layer of cells (e.g. roots 

 of most Angiosperms and 

 of some Vascular Crypto- 

 gams ; stems of Dipsa- 

 caceae, Valerianaceae, 

 Rubiacea?, Lobeliaceee, 

 Campanulaceae, Planta- 

 jgineae, Polemoniaceae, etc., 

 and generally of aquatic 

 Angiosperms) ; or of more 

 than one layer throughout 

 (roots of some Dicotyle- 

 dons, e.g. Vine, Diptero- 

 carpus, Ficus rubiginosa, 

 etc., and of Gymnos perms 

 generally ; commonly in 

 the stem and leaf -stalk) ; 

 or in part of one layer and 

 pai't of more than one (e.g. 

 root of some Ferns and Leguminosae). 



The pericycle may be homogeneous or heterogeneous; that is, it 

 may consist of the same kind of tissue throughout, or of several 

 kinds of tissue. The typical homop-eneous pericycle consists of 



FIG. 130. A transverse section of a young stem 

 of Aristolochia Sipho, illustrating the arrangement 

 of the primary tissues in a monostelic stem, in 

 which the vascular cylinder is hollow, enclosing a 

 pith. (After Strasburger : x 9). c Cortical ground- 

 tissue, with collenchyma cl j e endodermis ; pc peri- 

 cycle, continuous by means of interfascicular con- 

 junctive tissue (medullary rays) with the medullary 

 conjunctive tissue m (pith) ; sic ring of sclerenchy- 

 rna belonging to the pericycle ; fo vascular bundles 

 in an interrupted circle ; they are open and colla- 

 teral; cb bast; p protophloem; fc fascicular cam- 

 bium ; ifc interfascicular cambium ; vl wood ; the 

 central pointed end of each wood-bundle consists 

 of protoxylem, and the central ends of the whole 

 ring of wood-bundles constitute the medullary 

 sheath. 



