See also:

• ANATOMY
• ANATOMY (Gr. avaroyil, from ava-silo c', to cut up)

D, Part of branched filamentous thallus of the multicellular Green Alga Oedocladium. cr. ax., Green See also:

• AXIS (Lat. for " axle ")

This is in accordance with the See also:

• ORIGINAL

Embedded in the protoplasm are a number of See also:

• STARCH

V. Endodermal cell of Phanerogam, with suberized central See also:

• BAND

It is from such a living and assimilating cell, performing as it does all the vital functions of a green plant, that, according to current theory, all the different cell-forms of a higher plant have been differentiated in the course of descent. Among the Green Algae the differentiation of cells is comparatively slight. Many forms, even when multicellular, have all their cells identical in structure and See also:

• FUNCTION

In Caulerpa the See also:

• IMITATION (Lat. imitatio, from intitari, to imitate)

The cells belonging to any given thread may be recognized at an See also:

• EARLY
• EARLY, JUBAL ANDERSON (1816-1894)

According to a See also:

• LAW
• LAW (0. Eng. lagu, M. Eng. lawe; from an old Teutonic root lag, " lie," what lies fixed or evenly; cf. Lat. lex, Fr. loi)
• LAW, JOHN (1671—1729)
• LAW, WILLIAM (1686-1761)

In many Laminariaceae the thallus also grows regularly in thickness by division of its surface layer, adding to the subjacent permanent tissue and thus forming a secondary See also:

• MERISTEM (Gr. Aquas-6s, divided or divisible)

The internal tissue of the body of the solid higher Fungi, particularly the elongated stalks (stipes) of the fructifications of the Agarics, consists of hyphae running in a longitudinal direction, which no doubt serve for the conduction of organic food substances, just as do the " trumpet-hyphae," similar in appearance, .though not in origin, of the higher Brown Seaweeds. (In one genus (Lactarius) " See also:

• MILK (0. Eng. meoluc; from a common Indo-European root, cf. Lat. mulgere, Gr. ?t dXyetv)

But these two principles do not find their full expression till we come, in the ascending See also:

• SERIES (a Latin word from serere, to join)

In three genera—Blyttia, Symphyogyna and Hymenophytumthere are one or more strands or bundles consisting of long thick-walled fibre-like (prosenchymatous) cells, pointed at the ends and running longitudinally through the thick midrib. The walls of these cells are strongly lignified (i.e. consist of woody substance) and are irregularly but thickly studded with simple pits (see CYTOLOGY), which are usually arranged in spirals running round the cells, and are often elongated in the direction of the spiral (fig. i, I). These cells are not living in the adult state, though they sometimes contain the disorganized remains of protoplasm. They serve to conduct water through the thallus, the assimilating parts of which are in these forms often raised above the soil and are comparatively remote from the rhizoid-bearing (water-absorbing) region. Such differentiated water-conducting cells we See also:

• CALL (from Anglo-Saxon ceallian, a common Teutonic word, cf. Dutch kallen, to talk or chatter)

The latter are plates of green tissue one cell thick, while the stem consists of uniform more or less elongated cylindrical cells. The base of the stem bears numerous cell-filaments (rhizoids) which See also:

• FIX, THEODORE (180o-1846)

In the_aquatic, semi-aquatic, and xerophiloustypes, where the whole surface of the plant absorbs water, perpetually in the first two cases and during rain in the last, the hydrom strand is either much reduced or altogether absent. In accordance with the general principle already indicated, it is only where absorption is localized (i.e. where the plant lives on soil from which it absorbs its main supply of water by means of its basal rhizoids) that a water-conducting (hydrom) strand is developed. The leaves of most mosses are flat plates, each consisting of a single layer of square or oblong assimilating (chlorophyllous) cells. In many cases the cells bordering the leaf are produced into See also:

• TEETH (O.E. Eel); plural of tooth, O.E. top)

In addition to the water-conducting tissue or hydrom there is a well-developed tissue (leptom) inferred to be a conducting channel for organic substances. This leptom is not so highly differentiated as in the most advanced Laminariaceae, but shows some of the characters of sieve-tubes with great distinctness. Each leptoid is an elongated living cell with nucleus and a thin layer of protoplasm lining the wall (fig. i, K). The whole cavity of the cell is sometimes stuffed with proteid contents. The end of the cell is slightly swollen, fitting on to the similar swollen end of the next leptoid of the See also:

• ROW, JOHN (c. 1525—1580)

The midrib has a strong band of stereom above and below. In its centre is a band-shaped bundle consisting of rows of leptom, hydrom and amylon cells. This bundle is continued down into the cortex of the stem as a leaf-trace, and passing very slowly through the sclerelchymatous external cortex and the parenchymatous, starchy internal cortex to join the central See also:

• CYLINDER (Gr. KvAwSpos, from KvXivaety, to roll)

In Dawsonia superba, a large New See also:

• ZEALAND (also SEALAND Or SEELAND; Danish Sjaelland)

The gametophyte, which bears the sexual organs, is either a free-living thallus corresponding in degree of differentiation with the lower liverworts, or it is a mass of cells which always remains enclosed in a spore and is parasitic upon the sporophyte. The body of the sporophyte in the great See also:

• MAJORITY (Fr. majorite; Med. Lat. majoritas; Lat. major, greater)

This surface layer in the typically subaerial " shoot " of the sporophyte in Pteridophytes and Phanerogams is known as the epidermis, though the name is restricted by some writers, on account of developmental See also:

• DIFFERENCES, CALCULUS OF (Theory of Finite Differences)

The structure of the stomata of the sporophyte of vascular plants is fundamentally the same as that of the stomata on the sporogonium of the true mosses and of the liverwort Anthoceros. Stomata are often situated at the bottom of pits in the surface of the leaf. This arrangement is a method of checking transpiration by creating a still See also:

• ATMOSPHERE (Gr. fir µ6r, vapour; orkaipa, a sphere)

The cells of hairs may have living con-tents or they may simply contain air. A very common function of hairs is to diminish transpiration, by creating a still atmosphere between them, as in the case of the sunk stomata already mentioned. But hairs have a variety of other functions. They may, for instance, be glandular or stinging, as in the common stinging See also:

• NETTLE (0. Eng. netele, cf. Ger. Nessel)

Above each cavity is situated a so-called water-stoma, no doubt derived phylogenetically from an ordinary stoma, and enclosed by guard-cells which have nearly or entirely lost the See also:

• POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

It corresponds in function with the rhizoid of a Bryophyte. At the apex of a root, covering and protecting the delicate tissue of the growing point, is a special root-cap consisting of a number of layers of tissue whose cells break down into mucilage towards the outer surface, thus facilitating the passage of the apex as it is pushed between the particles of soil. The cortex, as has been said, is in its origin the remains of the primitive assimilating tissue of the plant, after differentiation Cortex. of the surface layer and the conducting system. It consists primitively of typical living parenchyma; but its differentiation may be extremely varied, since in the complex A B See also:

• FTC

It also always possesses a well-developed lacunar system communicating with the external air through stomata (in the young stem) or lenticels (see below). This lacunar system not only enables the cells of the cortex itself to respire, but also forms channels through which air can pass to the deeper lying tissues. The cortex of the older stem of the root frequently acts as a reserve See also:

• STORE (from O. Fr. estor or estoire, Late Lat. staurum or instaurum, stock, provisions, supply, from the late use of instaurare, to provide, properly to construct, renew, restore)

In the leafy shoot this function is mainly localized in the cortical tissue of the leaves, known as mesophyll, Mesophym which is essentially a parenchymatous tissue containing chloroplasts, and is penetrated by a system of intercellular spaces so that the surfaces of the assimilating cells are brought into contact with air to as large an extent as possible, in See also:

• ORDER
• ORDER (through Fr. ordre, for earlier ordene, from Lat. ordo, ordinis, rank, service, arrangement; the ultimate source is generally taken to be the root seen in Lat. oriri, rise, arise, begin; cf. " origin ")
• ORDER, HOLY

The cells of these sheaths are often distinguished from the rest of the mesophyll by containing little or no chlorophyll. Occasionally, however, they are particularly See also:

• RICH, BARNABE (c. 1540-1617)
• RICH, CLAUDIUS JAMES (1787-1821)
• RICH, JOHN (1692-1761)
• RICH, PENELOPE, LADY (c. 1562–1607)
• RICH, RICHARD, 1ST BARON RICH (1490?-1567)

Hence we must conclude that the conducting system of the Pteridophytes has had an entirely separate evolution. All the surviving forms, however, have a completely established double system with the specific characters alluded to, and since there is every reason to believe that the conditions of evolution of the primitive Pteridophyte must have been essentially similar to those of the Bryophytes, the various stages in the evolution of the conducting system of the latter (p. X32) are very useful to compare with the arrangements met with in the former. The hydroid of a Pteridophyte or of a Phanerogam is characteristically a dead, usually elongated, cell containing air and water, and Tissue either thin-walled with lignified (woody) spiral (fig. 1, P.) Elements. or See also:

• ANNULAR, ANNULATE

The leptoid of a Pteridophyte (fig. 1, o.) is also an elongated cell, with a thin lining of protoplasm, but destitute of a nucleus, and always in communication with the next cell of the leptom strand by perforations (in Pteridophytes often not easily demonstrable), through which originally pass strings of protoplasm which are bored out by a ferment and converted into relatively coarse " slime strings," along which pass, we must suppose, the organic substances which it is the special function of the leptoids to conduct from one part of the plant to another. The peculiar substance called callose, chemically allied to cellulose, is frequently formed over the surface of the perforated end-walls. The structure formed by a number of such cells placed end to end is called a sieve-tube (obviously comparable with a xylem-vessel), and the end-wall or See also:

• AREA

4). In the downward growing part of the axis (primary root), Arrange-however, the peripheral mantle of phloem is interrupted, See also:

• TENT

In the stele of the root the strands of tracheids along the lines where the xylem touches the pericycle are spiral or annular, and are the xylem elements first formed when the cylinder is developing. Each strand of spiral or annular first-formed tracheids is called a protoxylem strand, as distinct from the metaxylem or rest of the xylem, which consists of thick-walled tracheids, the pits of which are often scalariform. The thin-walled spiral or annular tracheae of the protoxylem allow of longitudinal stretching brought about by the active growth in length of the neighbouring living parenchymatous cells of a growing organ. During the process the thin walls are stretched and the turns of the spiral become pulled apart without rupturing the wall of the tracheid or vessel. If the pitted type of tracheal element were similarly stretched its continuously thickened walls would resist the stretching and eventually break. Hence such tracheae are only laid down in organs whose growth in length has ceased. The stele is called monarch, diarch, . . . polyarch according as it contains one, two, . . or many protoxylems. When the protoxylem strands are situated at the periphery of the stele, abutting on the pericycle, as in all roots, and many of the more primitive Pteridophyte stems, the stele is said to be See also:

• EXARCH (EEapxos, a chief person or leader)

When the protoxylems have an intermediate position the stele is mesarch (many Pteridophytes and some of the more primitive Phanerogams). In many cases external protophloem, usually consisting of narrow sieve-tubes often with swollen walls, can be distinguished from metaphloem. As the primitive stele of a Pteridophyte is traced upwards from the primary root into the stem, the phloem becomes continuous round the xylem. At the same time the Evolution stele becomes more bulky, all its elements increas- efthe See also:

• ING

Sometimes this See also:

• CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

In the latter case each segment of the solenostele frequently resembles a Dictyoste/y. haplostele, the segments of inner endodermis, pericycle, phloem and dt.