33.] CHAPTER II. THE TISSUES. 179 



other figure corresponding to the sectional outline of the member), 

 the orientation of the bundles in the stele, as indicated by the posi- 

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

 the transverse section of the member. For instance, in medullate 

 monostelic stems (Fig*. 130) the protoxylem forms the innermost 

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

 groups is sometimes specially designated the medullary sheath. 

 This condition also obtains in schizostelic members, as in the 

 rhizomes and aerial stems of Equisetum (see Fig. 116). In the 

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

 protoxylem is always outermost or peripheral, abutting on the 

 pericycle (Figs. 135-6). 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 protoxylem is a structure of considerable morphological 

 importance, serving as it does to mark the individuality of the 

 xylem-bundle of which it forms part. This means of distinction 

 is often of great use in determining the constitution of large masses 

 of primary vascular tissue, indicating whether they consist of one 

 bundle, or are gamodesmic, consisting of several fused bundles (e.g. 

 solid vascular cylinders of roots, stem of Lycopodium, vascular 

 strands of polystelic stems, etc.), and if the latter, of how many 

 bundles they consist. 



It has been customary to speak of such a gamodesmic vascular mass as a 

 single bundle describing it as diarch, triarch, tetrarch, etc., according to the 

 number of protoxylem-groups detected ; but this use of the terms is inaccurate. 

 Every xylem-bundle has but one protoxylem-group ; that is, it is monarch ; 

 wherever two or more protoxylem-grou-ps are to be found, they indicate the 

 fusion of a corresponding number of bundles ; in this sense the terms may 

 be convenient to describe the composition of masses of vascular tissue. 



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 root 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 



