Part II

structure intermediate between the collateral and the concentric bundle. The former type of concentric bundle occurs in the rhizomes of various Monocotyledons (Acorus, Iris, Cyperus, Carex, etc.), and in the medullary bundles of the stem of some Dicotyledons (Rheum, Statice, Ricinus, Piper, etc.). The latter type is rare in Phanerogams (e.g. the cortical and medullary bundles of the Melastomacese) ; but it prevails in the Filicinae and in Selaginella, when the gamodesmic bundles (two or more) of each stele of the polystelic stem, form a cen- tral mass of wood completely, or nearly completely, surrounded by a ring of bast. The relative posi- tion of the phloembundles and xylembundles ivhen they are distinct from each other is such that they alternate with each other so that a radius drawn t' Fi. 134. Transverse section of a concentric bundle, with external wood, from the rhizome of Iris (x 360): t tracheae ; t' protoxylem ; s sieve-tubes ; g companion-cells, of the internal bast. from the centre to 4he surface of the member cuts through either a phloem or a xylem-bundle, but riot through both (Fig. 136). This arrangement occurs only in monostelic members; it is common to all roots, and occurs in the stem of Lycopodium and Psilotum though in a less regular manner than in roots. It is commonly termed the radial arrangement. The Differentiation of the Primary Vascular Bundle. The first indication of the development of vascular tissue in the plerome is the differentiation of one or more strands of narrow elongated 176 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 33. merismatic cells, the procambium or primary desmogen (see p. 151) ; each procambium-strand of the plerome becomes a vascular bundle of the stele. The development of the vascular tissue does not take place simultaneously throughout the whole transverse section of the procambium-strand, but begins at one definite point, and extends in one or more directions from that point. The development of the xylem-bundle (or part of a conjoint bundle) begins with the differentiation of one or a few tracheids or tracheae, constituting the protoxylem ; the walls of the corre- sponding pro- cambium cells become spirally thickened and lignified, and the protoplasmic contents of the cells disappear. It is an important generalisation that spiral or annular vessels (or tracheides) are characteristic FIG. 135. Part of a transverse section of the stele of the Sar. saparilla-root (Smilax) : r cortex ; ed endodermis with passagecells tl; the pericycle and the interfascicular conjunctive tissue v are sclerenchymatous ; v' the pith ; x the protoxylem, and t a pitted vessel of a wood-bundle: s a bast-bundle. The alternation, or radiHl arrangement, of the wood and bast-bundles is shown, (x 300.) of, and abso- lutely confined to, the proto- xylem of the bundle. The remainder of the primary wood (i.e. the wood which is developed from the procambium) is then gradually differentiated, the walls of the tracheides or tracheae presenting one or other of the various kinds of pitted marking (p. 104). Similarly, the development of the phloem-bundle (or part of a conjoint bundle) begins with the differentiation of a small group of sieve-tubes and companion-cells, constituting the protohploem, which do not, however, differ in any marked manner from the 33.] CHAPTER II. THE TISSUES. 177 rest of the primary phloem, but their cavities soon become obliterated, so that they then look like strands of swollen cellwall (Fig. 137). The details of the differentiation of the primary vascular tissue are essentially the same as in the case of the secondary vascular tissue described on p. 202. The longitudinal differentiation of the primary vascular tissue does not take place in the same order in all cases. In roots, and in stems with cauline vascular tissue, the longitudinal differentiation proceeds acropetally. In stems with common bundles the differ- entiation usually begins in the procambium- strand at a node> proceeding both downwards in the internode of the stem, and outward into the young leaf. 371 In the majority of in- stances, the ivhole of the procambium - strand becomes differentiated into permanent tissue, either wood or bast this is true ; for all roots, and for the stems of nearly all Pteridophyta and Monocotyledons (Fig. 137). Bundles of this kind are said to be closed. Ill the Stems of most G.yimnnosperms and i , Dico- ,, tyledons, Oil the Other hand, the whole of the procambium is not COIl- FlG- 136. Transverse section of the central part of the root of Atoms Calamus (after Strasburger : x 90) ; c lacunar cortex; e endodermis ; p pericycle; Sprimarywood.bundle8. witathe small spiral vessels m (protoxylem) externally; i? bast- bundles ; ^ arrangement of the bundles is radial. pith 5 verted into the primary wood and bast of the collateral conjoint bundle, but a portion of it persists as an embryonic merismatic tissue, the cambium, forming a transverse zone between the wood on the inner (central) side and the bast on the outer side (see Figs. 130, 139). Such a bundle is said to be open* . Some few Dicotyledons have closed bundles (i.e. no cambium) in the stem, e.g. Adoxa, Ranunculus Ftcaria, Nymphseacese, Myriophyllum, Utricularia, etc. The position of the protoxylem and of the protophloem in the transverse section of the bundle is not the same in the different members V. S. B. N 178 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 33. The protophloem is in all cases external : and though the protoxylem is also generally external, it is sometimes internal (as in the bundles in the petiole of Cycads, in the stem of Isoetes, and in the concentric bundles of stems and petioles of many Ferns), being more or less surrounded by the rest of the primary xylem. FIG. 137. Transverse section of a conjoint, collateral, closed, vascular bundleof the stem of a Monocotyledon (Zea Mais) : a outer or peripheral end of the bundle ; t inner or central end; p conjunctive tissue, the portion immediately investing the bundle being sclerenchy- matous ; I lyeigenous intercellular space ; s r spiral and annular vessels constituting the protoxyhm; g g large pitted vessels, between which lie the smaller pitted vessels of the wood ; v v v sieve-tubes of the bast with intervening companion-cells ; just outside the bast, and within the sclerenchymatous sheath, the remains of the protophloem are visible. (After Sachs: x563.) In members, whether monostelic or polystelic, in which the primary bundles or the steles are arranged in one or more circles (or* 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 position 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 protoxylemgroups 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 protophloem 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 considerably 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 180 PART II. THE INTIMATE STRUCTURE OF PLANTS. 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- dles, each of the bundles of the root bifurcating above, so that there are commonly twice as many bundles in the stem as in the corresponding root. The structure of the primary bundle. The primary wood (whether FIG. 138. Radial longitudinal section of a conjoint, closed, collateral bundle from tbo stem of a Monocotyledon (Zea Mais ; after Strasburger, x 180) ; to the right is the central (medullary) limit of the bundle ; to the left the peripheral (cortical) limit ; c p protophloem j v sieve-tubes of the bast, with companion-cells s ; sp a a1 , the protoxylem ; a a' remains of ruptured annular vessel lying in the lysigenous lacuna I ; vg sheaths of sclerenchymatous conjunctive tissue. (Compare this with Fig. 137.) in an isolated or a conjoint bundle) consists essentially of lignified tracheal tissue (tracheae, or tracheids, p. 134), together with a varying proportion of wood-parenchyma, more or less lignified, the cells being occasionally somewhat fibrous. The protoxylem (see p. 176) is usually a conspicuous feature; in transverse section, on account of the relative smallness of its tracheae (or tracheids) ; in 33.] CHAPTER II. THE TISSUES. 181 longitudinal section, on account of the loose spiral or annular thickening's of their walls. The looseness of the spiral or annular markings is due to the fact that these vascular cells are the first formed constituents of the bundles, and that consequently they are considerably stretched by the continuance, for a time, of the growth in length of the adjacent undifferentiated tissues ; hence the successive thickenings become more or less widely separated, and the wall of the vessels may be torn and destroyed (Fig. 138). The primary bast or phloem consists essentially of sieve-tissue (p. 135) and of parenchyma. The sieve-tissue consists in all cases mainly of sieve-tubes of simple structure (Fig. 97, p. 136), con- stituting the vascular tissue of the bast, with which companion- cells are associated in Angiosperms but not in Gymnosperms and Pteridophyta. In some Angiosperms, particularly in the closed bundles of Monocotyledons (Fig. 137), there is no bast- parenchyma, the whole bast consisting of sieve-tubes and com- panion-cells : but this tissue is generally present, and is readily distinguishable from the companion-cells by the larger size of its cells. In some cases (e.g. some Palms) the bast-parenchyma is to some extent replaced by sclerenchymatous fibres ; otherwise the occurrence of fibres in the primary bast is rare. The cambium is present in the collateral primary bundles of the stem of most Gymnosperms and Dicotyledons ; it is never present in primary bundles of any other type of structure ; nor, on the other hand, is it always present in a collateral bundle (absent in Equisetum, Monocotyledons, some herbaceous Dicoty- ledons, see p. 177). It lies between the bast externally and the wood internally, and consists essentially of a single layer of merismatic embryonic cells rich in protoplasmic contents, and with walls of cellulose. In transverse section (see Fig. 133) the cells are oblong, with their longer axes placed tangentially ; in longitudinal section (Fig. 153) the cells are seen to be elongated and somewhat prosenchy- matous, like the procambium- cells, where they abut on the wood or on the bast ; but where they abut on primary medullary rays they are short and parenchymatous. Variations in the structure of a bundle. It is mentioned on p. 173 that the leaf-trace bundles of Palms thin out as they curve out- wards towards the pericycle in the lower part of their course ; the bundle is, in fact, thickest where it is most deeply placed in the stem ; the difference in size of the bundle, as seen in trans- 182 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 33. verse section, is due a variation in the number, and to some extent also, in the size of its constituent elements, more especially in the wood. The variation in structure of a vascular bundle can be well observed in connexion with the foliage-leaf. The number of bundles entering the leaf (petiole, when present) from the stem FIG. 139. .4 Transverse section of an open conjoint, collateral, vascular bundle in the M stem of the Sunflower. X Pith. Xylem. C Cambium. P Phloem. R Kortex ; s Small, and ' large spiral vessels (protoxylem) ; t pitted vessels; t' pitted vessels in course of formation from the cambium; 7i wood-fibres; sb sieve tubes; b fibres of tHe hetero- geneous pericycle ; e endodermis or bundle-sheath ; ic inter-fascicular conjunctive tissue. B Radial vertical section through a similar bundle (somewhat simplified) lettered like the former. (x!50). varies from one to many ; these bundles, when derived from a monostelic stem, are segments or branches of the stele (meristeles), , 33.] CHAPTER JI. THE TISSUES. 183 when derived from a polystelic stem, they are entire steles. The structure of the leaf-bundles corresponds essentially with that of the stem-bundles if the latter are concentric or collateral, etc., so ; are the former, as a general rule ; but in the Cycads the bundles in the petiole have the protoxylem central next to the bast (see p. 178) instead of in the normal position which it occupies in the stem- bundles ; and again, whilst the bundles in the' petiole of most Ferns are, like those of the stem, concentric, in the ribs of the lamina they become collateral. At the same time it should be pointed out that whilst the general relations of the bundle are usually the same in both stem and leaf^ the changed conditions usually involve a somewhat different description. For instance, the common conjoint bundles of the stem of a Dicotyledon are collateral, the protoxylem being the most internal or central part of the bundle, the protophloern the most external or peripheral: part ; on tracing a bundle into a leaf which is dorsi ventral, and lies in a nearly horizontal plane of expansion, the xylem of the conjoint bundle will be found to lie towards the upper (ventral) surface of the leaf, with the protoxylem uppermost, whilst the phloem is directed towards the lower (dorsal) surface, with the protophloem lowermost ; the description of the position and rela- tions of the bundle must be in accordance with the symmetry of the member of which it forms part. The Termination of the Vascular Bundle. The gradual thinning out and termination of the vascular bundle can nowhere be more satisfactorily studied than in leaves. The bundles, when traced towards their ultimate ramifications, are seen to diminish in bulk in consequence, partly, of a reduction in number of the constituent elements, and partly also to the smaller size of the elements which still remain. The mode of termination of the vascular bundles in foliage-leaves is briefly as follows. In many cases the bundles have only free ends, as in most Pteridophyta (e.g. Adiantum, Selaginella), and generally in small reduced leaves. In others, there are no free ends, but the finer branches anastomose with each other to form a closed system ; this is characteristically the case where the venation is parallel (e.g. Monocotyledons, see p. 55). In others, again, the finer branches anastomose, forming a network from the meshes of which the ultimate branches project among the mesophyll-cells as free ends : this obtains generally among Dicotyledons. The free ends of the bundles consist of one or two rows of short tracheids with close spiral markings; no sieve- tubes can 184 PART II. THE INTIMATE STRUCTURE OF PLANTS. 34. [ be traced quite to the extremity ; they disappear further back, and their place is taken by parenchymatous cells. Bundles often terminate in connection with glandular tissue ; for instance, in chalk-glands (see Fig. 100, p. 139), or nectaries. 34. Histology of the Development of Secondary (Vjembers. It has been already pointed out (p. 17) that the growing- point is the seat of development, not only of new tissue, but also of new members ; and further (p. 19), that secondary members are developed either by dichotomy or by lateral out- growth . A. Development of normal branches of the shoot or of the thallus. only takes place at the growing-point, whether apical or inter- calary. a. By dichotomy. This only occurs in apical growing-points ; two modes may be distinguished accordingly as the growing-point has or has not an apical cell : 4 FIG. 140. A B C successive stages in true dichotomous branching by longitudinal division of an apical cell ; from the shoot of Dictyota dichotoma (highly magnified ; after Naegeli). wlien there is an apical cell, true dichotomous branching is effected by the longitudinal division of the apical cell into two, each of which becomes the apical cell of a branch : A spurious form of dichotomy occurs in some plants (e.g. thalloid Junger- mannieae) ; here, though the apical cell of the branch is developed from a segment of the apical cell of the main shoot, yet since the really lateral branch grows quite as vigorously as the main shoot, the result is an apparently dichotomous branching. when there is no apical cell, the growing-point becomes broadened, and the central portion of it passes over into condition of permanent tissue, leaving two distinct masses of embryonic tissue, which constitute the growing-points of the two branches (e.g. March antiaceee). b. By lateral outgrowth ; this may occur in either an apical or intercalary growing-point : 34] CHAPTER II. THE TISSUES. 185 when there is a single initial cell in the growing-point, the growing-point of the branch is developed either directly from the initial cell itself, as in some Algae (Fig. 141 A), or more com- monly from a segment of the initial cell, as in many Algae {Fig. 141 (7), Mosses, Liverworts, etc. : when there is not a single initial cell (e.g. Phanerogams), the growing-point of the branch is formed by division of cells of the periblem, including several layers, which grow and divide, forming a lateral protuberance with the growth of which the dermatogen keeps pace ; the primary meristem of the branch undergoes differentiation into tissue-systems corresponding to those of the parent members, and continuous with them. B. C. FIG. 141. Illustrating development of lateral members; A (Stypocaulon scoparium, x30): B from the apical cell itself : (Desmarestia ligulata, x 60) from the segments of an intercalary growing-point ; C (Chcetopteris plumosa, x 40) from the segments of the apical cell. (After Falkenberg). Normal branches, however the details of their development may vary, agree in this, that they are, with rare exceptions (see p. 20), of exogenous origin. B. Development of Leaves only takes place at the growing-point of a stem, and always by lateral outgrowth (see p. 45). When the growing-point of the stem has a single initial cell, the growing-point of the leaf is developed either from the apical cell itself, or, more commonly, from the whole or a part of a segment of the apical cell. When the growing-point of the stem has not a single initial cell, as in Phanerogams, the growing-point of the leaf is formed by the division of cells belonging to one or more of the superficial layers of the periblem, accompanied by growth and division of the cor- responding cells of the dermatogen. 186 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 34. The primary meristem of the leaf becomes differentiated into tissue-systems corresponding to, and continuous with, those of the stem which bears it. In the developing leaves of those vascular plants which have common bundles (see p. 177), the differentiation of the protoxyleni begins at the point of junction of leaf and stem, extending outwards in the procambium-strands of the leaf, and inwards in those of the stem. The development of secondary branches of the leaf takes place in essentially the same manner as that of the leaf from the stem. Dichotomous branching of the leaf (see p. 51) takes place in the same way as dichotomous branching of the stem. It will be seen that the development of a leaf on any stem takes place in essentially the same way as the development of a lateral branch on that stem ; it is only later that leaves and branches assume their distinctive characters. C. Development of Branches of the Root. It has been pointed out that the only normal secondary members produced by the root are root-branches, or secondary roots ; these may be developed either by dichotomy or by lateral outgrowth. a. By dichotomy. This has only been observed in certain spor- phytes among the Pteridophyta (Lycopodium, Isoetes). Here the growing-point broadens, under the root-cap, the central portion passing over into permanent tissue, whilst the two sides remain merismatic and form the growing-points of the two secondary roots ; the old root-cap is exfoliated, and each growing-point forms a new one for itself. The successive dichotomies take place in planes at right angles to each other. b. By lateral outgrowth. It has been already stated (p. 62) that the lateral development of secondary members does not take place at the growing-point of the root, but at a considerable distance behind it, where the tissues have already assumed their permanent differentiation. The lateral roots are developed endogenously from a layer of this tissue which remains embryonic longer than the adjacent tissues. This layer may be either the pericycle, as in Phanerogams, or the endodermis, as in most Vascular Cryptogams. In the Phanerogams (Fig. 142), the growing-point of a lateral root is formed by the growth and division of a group of pericycle- cells, lying usually just externally to the outer end of a xylem- bundle ; hence there are as many longitudinal rows of lateral roots produced as there are xylem-bundles in the parent root, and cor- 34.] CHAPTER II. THE TISSUES. 187 responding with them in position. But to this rule there are some exceptions ; for instance, when, as in the Grasses and Cyperacese, the pericycle is wanting opposite the xylem-bundles, the lateral roots are developed, not opposite to the xylem-bundles, but opposite to the phloem-bundles ; again, when there are only two xylem- bundles in the parent root, four rows of lateral roots are produced each root being developed on one side of a xylem- bundle of the A parent root. similar displacement occurs in Umbel 1 iferce, Araliaceae and Pittosporeas, where the pericycle