Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/103

this mass is termed the procambium of the fibro-vas- culur bundle. As growth proceeds changes take place in the cells by thickening, and, their contents disappear ing, various kinds of cells and vessels are formed. In this way the whole mass of the procambium may be con verted into permanent tissue, and then growth ceases ; or an inner portion of ths bundle remains merismatic, which is called the cambium, and then growth proceeds, the shoot or root increasing in thickness by the cambium forming new cells on both sides In the former case the bundle is closed, as in Cryptogams and Monocotyledons and some Dicotyledons ; in the latter it is open, as in most Dicotyle dons and Conifers. In every fibro-vascular bundle a separation into two groups of structures may be distin guished the wood or xylem layers, and the bast or phloem layers. As long as the bundle is open and cambium present, these layers are separated by the cambium (fig. 55). Their relative position as regards the axis of the stem or root varies. In some cases the phloem is nearer the circum ference, in other instances the xylem is peripheral, and in rare cases amongst Dicotyledons there are phloem layers both on the outer and inner sides of the fibro-vascular bundles. In vascular Cryptogams the phloem layers

always surround the xylem portion of the bundle.

Fig. 55.

Transverse section of an open fibro-vascular bundle, c, cambium; cb, continua tion of cambium between the fibro-vascular bundles ; y, large pitted vessels; t smaller pitted vessels and spiral vessels intermixed with wood-cells; y, inner phloem layers; 6, bast fibres; m is the parenchyma of the pith; r is the cortical parenchyma. Immediately external to the bark lies the bundle sheath of cells filled with starch. (Sachs.)

56.

I.o: .fjitudi:i:il section of nn open fibro-vnscular bundle, c. cambium ; 5. s , spiral vessels with fibres which can be unrolled; /, scalarifonn reticulated vessel; h, / , h", h ff , wood-cells ; t, bordered pitted vessel ; t f , young pitted vessel ; p, inner phloem layer; b, bast layer; ys, bundle-sheath; r, cortical parenchyma; m, pith parenchyma. The elements are developed from s to t in the xylcin portion. (Sacftt.)

In the xylem or wood portion of a fibro-vascular bundle the cells all tend to thicken their walls, and, consequently, numerous kinds of cells and vessels are found, which are usually arranged in a definite manner. On the side furthest removed from the phloem we find spiral vessels interspersed with wood -cells (figs. 55 and 56), outside are reticulated and scalariform vessels, and then interspersed with wood cells are large pitted vessels. In the phloem or bast layers the cells have not such a tendency to thicken, but usually remain thin-walled, forming ordinary parenchy- matous cells, or becoming perforated and forming sieve cells. In certain layers, how ever, the cell-walls are thick ened so as to become flexible, constituting the bast fibres. Around every fibro-vascular bundle a single layer of cells of the fundamental cellular tissue of the stem is marked off from its surroundings, the Cells get filled with starch- grains, and this constitutes what has been termed the bundle sheath or starch-bear ing layer (fig. 57). In fig. 57 is seen a transverse sec tion of the closed fibro-vascu lar bundle from the Maize , and it will be observed that it is essentially the same as that of the open bundle (fig. 55), only that all the cambium cells have passed into per inanent tissue.

Fig. 57.

cular bundle, r, annular vessel ; s, spiral vessel: I, inter-cellular canal; >7, pitted vessel, r,r, cambiforni tissue which has become permanent. Between v and j are reticulated vessels, p, sur rounding parenchyma. Outer cells a of the bundle are parenchymatous,t marks the inner side of the bundle. (Sachs.)

In all plants a provision is made for branching of the various organs, and two prin cipal forms of it may be recognized. In one of these Tra c s - t!on of a ck the generating axis elongates at the apex, producing in suc cession lateral structures. To this form the term monopo- tlium has been applied. In the second form there is a cessation of growth at the apex in the direction of previous elongation of the axis, and a con tinuance in two diverging directions. This is dichotomous branching. In their rudimentary state all branchings may be easily referred to one or other of those types ; but in the mature system it is frequently difficult to recognize the type, owing to irregular development of the successive branching. Thus in a dichotomous branching only one of the secondary axes may develop strongly, the weaker branch appearing as a small lateral shoot from its base; and an apparent primary shoot is thus produced which in reality consists of the bases of single branches of consecutive forkings. Such an axis is termed a pseudaxis or sijmpodium. And, again, in monopodial branchings the primary axis may continue to develop more strongly than its lateral axes, which in their turn develop similarly, and a racemose form arises ; or the primary axis may be arrested in growth, and the secondary axes develop more strongly and overlap it, when a cijmose branching results. More will be said on this subject when considering the inflorescence of Phanerogams, in which the various forms of branching are well seen.