128 PART II. ANATOMY AND HISTOLOGY. [ 33 



Some few Dicotyledons have closed bundles (i.e. no cambium) in the stem, 

 e.g. Adoxa, Ranunculus Ficaria, Nymphseacese, Myriopliyllum, Utricularia. 

 etc. 



The position of the protoxylcm and of the protophloem in the 

 transverse section of the bundle is not the same in the different 

 members. The protophloem is in all cases superficial : and though 

 the protoxylem is also generally superficial, it is sometimes in- 

 ternal (as in the bundles in the petiole of Cycads, in the stem of 

 Isoetes, and in the concentric steles of stems and petioles of many 

 Ferns), being more or less surrounded by the rest of the primary 

 xylem. 



In members, whether monostelio or polystelic, in which the 

 primary bundles or the steles are arranged in one or more circles 

 (or other figure corresponding to the sectional outline of the mem- 

 ber), the orientation of the bundles in the stele, as indicated by the 

 position of the protoxylem, bears a definite relation to the symmetry 

 o^ the transverse section of the member. For instance^ in medullate 

 monostelic stems (Fig. 97) the protoxylem forms the innermost 

 or central portion of the bundle ; the broken circle of protoxylem- 

 groups is sometimes specially designated the medullary sheath. 

 In the root, whether the vascular cylinder be medullate or not, the 

 protoxylem is always outermost or peripheral, abutting on the 

 pericycle (Fig. 102). This is also the case in monostelic stems 

 which are not medullate (e.g. stem of Lycopodium). The proto- 

 phloem is always external, abutting on the pericycle._ 



The transition from the root to the stem. Inasmuch as, generally 

 speaking, the type of primary structure of the root differs so con- 

 siderably from that of the corresponding stem, the transition from 

 the one to the other is a matter of some importance. Taking as an 

 illustration the case of a plant with a monostelic stem, the passage 

 from the radially arranged separate bundles of the primary rooj to 

 the collateral conjoint bundles of the stem is effected on this wise : 

 generally speaking, on tracing the wood- and bast-bundles of the 

 root upwards into the stem, the wood-bundles are found to twist 

 on themselves so that the protoxylem of each bundle, from being 

 peripheral in the root, comes to be central in the stem ; at the same 

 time they change their position somewhat so that they come to lie 

 on the same radii as the bast-bundles, or the bast-bundles may also 

 deviate somewhat from their straight course, and thus the conjoint 

 collateral bundles come to be constituted. As a rule, these changes 

 of position are accompanied by an increase in number of the bun- 



