118 



PART II. ANATOMY AXD HISTOLOGY. 



polystelic, one or more complete steles enter the petiole of the leaf 

 which is, consequently, either monostelic or polystelic. ~lVEen 

 the stem is monostelic, each leaf receives a portiqn^Jermed a 

 meristefc, of the stele of the stem; thisjneristelejnaj be either 

 entire, or be split up into a number of parts, each of which may 

 consist of but a single vascular bundle. 



The Conjunctive Tissue. The morphology of the conjunctive 

 tissue of the stele varies somewhat in accordance with the develop- 

 ment of the vascular tissue. When a solid vascular cylinder is 

 produced, there may be no conjunctive tissue at allj the whole of 

 t he plerome having developed into vascular tissue ; or the conjunc- 

 tive tissue may be limited to one or more peripheral layers, th.e 



pericycle, investing the 

 vascular cylinder ; or, 

 again, in addition_J,o the 

 pericycle, the Conjunct i ve 

 tissue may extend inwards 

 to some extent between 

 the bundles (interfascicu- 

 lar) of the stele. On the 

 other hand, when the vas- 

 cular cylinder is hollow 

 (see Fig. 97). the central 

 space is occupied by me- 

 dullary conjunctive tis- 

 sue, constituting the }>ith, 

 and connected with the 

 pericvcle by interfascicti- 

 lar conjunctive tissue con- 

 stituting the medullary 

 rays. Pith and medullary 

 rays are generally absent 

 from the steles of a poly- 

 stelic member. 



FIG. 97. A transverse section of a young stem 

 of Arixtolochia Sipho, illustrating the arrangement 

 of the primary tissues in a monostel'c stem, in 

 which the vascular cylinder is hollow, enclosing a 

 pith (after Strasburger: x 9) : c cortical tissue, 

 with collenchyma cl ; e endodennis ; pc pericycle, 

 continuous by means of interfascicular conjunctive 

 tissue (medullary rays) with the medullary con- 

 junctive tissue m (pith) ; sfc ring of sclerenchyma 

 belonging to the pericycle ; fv vascular bundles in 

 an interrupted circle ; they are open and collateral ; 

 cb bast ; p protophloem ; fc fascicular cambium ; 

 /c interfascicular cambium ; t>l wood ; the central 

 pointed end of each wood-bundle consists of 

 protox.ylem, and the central ends of the whole ring 

 of wool-bundles constitute the medullary sheath. 



A remarkable form of tis- 

 sue is that which invests 

 the two vascular bundles in 

 the acicular leaves of Pinus, 



and, to a less degree, of other Conifers (Fig. 93). The tissue consists of 

 parenchyma with some fibrous sclerenchyma: in the parenchyma two 

 special kinds of cells can be distinguished, which constitute what is some- 



