L, Optical section of cell of parenchyma in the same moss.
The term parenchyma is applied to tissues whose cells are isodiametric or cylin.drical in shape, prosenchyma tissues consisting of long narrow cells, with pointed ends.
By the formation of numerous cross-walls the resemblance to parenchyma is increased.
The group, the exceptions being met with almost entirely among the higher Brown Seaweeds, in which is found parenchyma produced by the segmentation of an apical cell of the whole shoot, or by cell division in some other type of meristem.
It always consists of true parenchyma, and is entirely formed by the cutting off of segments from an apical cell.
This passes gradually into the thinner-walled parenchyma of the inner cortex.
The whole of the cortex, stereom and parenchyma alike, is commonly living, and its cells often contain starch.
Associated with the conducting parenchyma are frequently found hydroids identical in character with those of the central strand of the stem, and no doubt serving to conduct water to or from the leaf according as the latter is acting as a transpiring or a waterabsorbing organ.
Frequently, also, a considerable differentiation of vegetative tissue occurs in the wall of the spore-capsule itself, and in some of the higher forms a special assimilating and transpiring organ situated just below the capsule at the top of the seta, with a richly lacunar chlorophyllous parenchyma and stomata like those of the wall of the capsule in the Anthocerotean liverworts.
Consists primitively of typical living parenchyma; bu its differpotlistion mov he esctremelv vsred, sinr-p in the rnmnle~
In other cases it does not differ histologically from the parenchyma of the rest of the cortex, though it is often distinguished by containing particularly abundant starch, in which case it is known as a starch sheath.
Associated with it are other tissues, consisting of parenchyma, mainly starchy, and in the Phanerogams particularly, of special stereom.
This is a morphological term given to the particular~ type of hydrom found in both Pteridophytes and Phanerogams, together with the parenchyma or stereom, or both, included within the boundaries of the hydrom tissue strand.
The sieve-tubes, with their accompanying parenchyma or stereom, constitute the tissue called phloem.
The sieve-tubes differ, however, from the tracheids in being immediately associated, apparently constantly, not with starchy parenchyma, but with parenchymatous cells, containing particularly abundant proteid contents, which seem to have a function intimately connected with the conducting function of the sieve-tubes, and which we may call proteid-cells.
When there is a single protoxylem strand in the centre of the stele, or when, as is more commonly the case, there are several protoxylem strands situated at the internal limit of the xylem,, the centre of the stem being occupied by parenchyma, the stele is endarch.
In others a central parenchyma or primetive pith a new region of the primitive stelar conjunctiveappears in the centre of the xylem.
Metu starchy xylem-parenchyma, which, when the xylem is bulky, usually appear among the tracheids, the phloem also often being penetrated by similar bands of phloem-parenchyma.)
As a bundle is traced towards its blind termination in the mesophyll the peridesmic stereom first disappears, the sieve-tubes of the phloem are replaced by narrow elongated parenchyma cells, which soon die out, and the bundle ends with a strand of tracheids covered by the phloeotermic sheath.
Sometimes all the parenchyma within the stele undergoes this change.
Sometimes development stops altogether, and a layer of undifferentiated parenchyma (the mesodesm) is left between them; or it may continue indefinitely, the central cells keeping pace by their tangential division with the differentiation of tissue on each side.
A tissue mother-cell of the xylem may, in the most advanced types of Dicotyledons, give rise to(I) a tracheid; (2) a segment of a vessel; (3) a xylem-fibre; or (4) a vertical file of xylem-parenchyma cells.
A tissue mother-cell of the phloem may give rise to (i) a segment of a sieve-tube with its companion cell or cells; (2) a phloem fibre; (3) a single phloem-parenchyma (cambiform) cell, or a ve~rtical file of short parenchyma cells.
When a given initial cell of the cambium has once begun to produce cells of this sort it continues the process, so that a radial plate of parenchyma cells is formed stretching in one plane through the xylem and phloem.
The xylem and phloem parenchyma consist of living cells, fundamentally similar in most respects to the medullary ray cells, which sometimes replace them altogether.
The parenchyma is often arranged in tangential bands between the layers of sievetubes and tracheal elements.
The xylem parenchyma is often found in strands associated with the tracheal elements.
The fibres belong to the same n,orpholcgical category as the parenchyma, various transitions being found between them; thus there may be thin-walled cells of the shape of fibres, or ordinary fibres may be divided into a number of superposed cells.
These intermediate cells, like the ordinary parenchyma, frequently store starch, and the fibres themselves, though usually dead, sometimes retain their protoplasm, and in that case may also be used for starch accumulations.
They are nearly always aggregated in strands, which, like those of the parenchyma, are not isolated, but are connected with one another.
Among Gymnosperms the secondary xylem is similarly simple, consisting of tracheids which act as stereom as well as hydrom, and a little amylom; while the phloem-parenchyma sometimes undergoes a differentiation, part being developed as amylom, part as proteid cells immediately associated with the sieve-tube, in other cases the proteid cells of the secondary phloem do not form part of the phloem-parenchyma, but occupy the top and bottom cellrows of the medullary rays, the middle rows consisting of ordinary starchy cells.
Characeae are separated from other Chlorophyceae by Ulvaceae that there is any pretension to the formation of a true a long interval, and present the highest degree of differentiation of parenchyma within the limits of the Chlorophyceae.
The typical foliage leaf consists of several layers, and amongst vascular plants is distinguishable into an outer layer (epidermis) and a central tissue (parenchyma) with fibro-vascular bundles distributed through it.
In leaves having a very firm texture, as those of Coniferae and Cycadaceae, the cells of the parenchyma immediately beneath the epidermis are very much thickened and elongated in a direction parallel to the surface of the leaf, so as to be fibre-like.
These constitute a hypodermal layer, beneath which the chlorophyll cells of the parenchyma are densely packed together, and are elongated in a direction vertical to the surface of the leaf, forming the palisade tissue.
In skeleton leaves, or leaves in which the parenchyma is removed, this arrangement is well seen.
In some cases there is only a network of filament-like cells, the spaces between which are not filled with parenchyma, giving a skeleton appearance to the leaf, as in Ouvirandra fenestralis (Lattice plant).
In both simple and compound leaves, according to the amount of segmentation and the mode of development of the parenchyma and direction of the fibro-vascular bundles, many forms are produced.
Of parenchyma, like the palm of the hand, as in the sycamore, castoroil plant, &c. The divisions of leaves with radiating venation may extend to near the base of the leaf, and the names bipartite, tripartite, quinquepartite, &c., are given according as the partitions are two, three, five or more.
When the development of parenchyma is such that it more than fills up the spaces between the veins, the margins become wavy, crisp or undulated, as in Rumex crispus and Rheum undulatum.
By a deficiency in development of parenchyma and an increase in the mechanical tissue, leaves are liable to become hardened and spinescent.
The moss-like covering of the "bedeguars" of the wild rose, the galls of a Cynipid, Rhodites rosae, represents leaves which have been developed with scarcely any parenchyma between their fibro-vascular bundles; and the " artichoke-galls " or " oak-strobile," produced by Aphilothrix L., which insect arrests the development of the acorn, consists of a cupule to which more or less modified leaf-scales are attached, with a peduncular, oviform, inner ga11.4 E.