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 OF PLANTS The second type of differentiation is that between supporting axis and assimilating appendages. The cells of the axis are commonly stouter and have much less chlorophyll than those of the appendages. This differentiation is parallel with that between stem and leaf of the higher plant. In the group of the Siphonese both these types of differentiation may exist in the single, long, branched, tube - like, and multinucleate cell which here forms the plant-body. Protosiphon (Fig. 1 B) is an example parallel with (Edodadium ; Bryopsis, with Draparncddia. In Caulerpa the imitation of a higher plant by the differentiation of fixing, supporting, and assimilating organs (root, stem, and leaf) from different branches of the single cell is strikingly complete. In the Siphoneous family of Codiacece (Fig. 1 E) the branches of the primitive cell become considerably interwoven one with another, so that a dense tissue-like structure is often produced. In this we get a further differentiation between the central tubes (branches of the primitive cell), which run in a longitudinal direction through the body, possess little or no chlorophyll, and no doubt serve to conduct food substances from one region to another, and the peripheral ones, which are directed perpendicularly to the surface of the body, ending blindly there, contain abundant chlorophyll, and are the assimilating organs. None of the existing Red Seaweeds has a unicellular body. The thallus in all cases consists of a branched filament of cells placed end to end, as in many of the Green Algse. Each branch grows simply by the transverse division of its apical cell. The branches may be quite free or they may be united laterally to form a solid body of more or less firm and compact consistency. This may have a radial stem-like organization, a central cell-thread giving off from every side a number of short sometimes unicellular branches, which together form a cortex round the central thread, the whole structure having a cylindrical form which only branches when one of the short cell-branches from the central thread grows out beyond the general surface and forms in its turn a new central thread, from whose cells arise new short branches. Or the thallus may have a leaf-like form, the branches from the central threads which form the midrib growing out mainly in one plane and forming a lamina, extended right and left of the midrib. Numerous variations and modifications of these forms exist. In Fig. 1.—Examples of the Differentiation of Tissue in Plants. case, while the internal threads which bear the cortical A, Cell (individual) of the unicellular Green Alga Pleurococcus, as an any example of an undifferentiated autonomous assimilating cell, pr., Cell proto- branches consist of elongated cells with few chromatophores, and no doubt serve mainly for conduction of food substances, the plasm; n., nucleus; chi., chloroplast; c.w., cell-wall. B, Plant of the primitive Siphoneous Green Alga Protosiphon hotryoides. superficial cells of the branches themselves are packed with The primitive cell sends colourless tubelets (rhizoids, rh.) into the mud on chromatophores and form the chief assimilating tissue of the which it grows. The subaerial part is tubular or ovoid, and contains the plant. In the bulky forms colourless branches frequently grow chloroplast (chi.). There are several nuclei. C, Base of the multicellular filamentous Green Alga Chcetomorpha cerea. out from some of the cortical cells, and, pushing among the The basal cell has less chlorophyll than the others, and is expanded and fixed threads in a longitudinal direction, serve to firmly to the rock on which the plant grows by the basal surface, rh., thus already-formed strengthen the thallus by weaving its original threads together. forming a rudimentary rhizoid. D, Part of branched filamentous thallus of the multicellular Green Alga The cells belonging to any given thread may be recognized at an CEdocladium protonema. cr. ax., Green axis creeping on the surface of damp stage of growth, because each cell is connected with its soil; rh., colourless rhizoids penetrating the soil; asc. ax., ascending axes of early neighbours belonging to the same thread by two depressions or green cells. E, Vertical section of frond of the complicated Siphoneous Green Alga pits, one at each end. The common wall separating the pits of Halimeda. The substance of the frond is made up by a single much-branched the two adjoining cells is pierced by strands of protoplasm. The tube, with interwoven branches, cond. med., Longitudinally running com- whole structure, consisting of the two pits and the wall between, paratively colourless central (medullary) branches, which conduct food substances and support the (ass. cor.) green assimilating cortical branches, which is known as a genetic pit. Other pits, connecting cells not beare the ends of branches from the medulla and fit tightly together, forming longing to the same branch, are, however, formed at a later stage. the continuous surface of the plant. Many of the lower forms of Brown Seaweeds have a thallus F, Section through the surface tissue of the Brown Alga Cutleria multifida, showing the surface layer of assimilating cells densely packed with phaso- consisting of simple or branched cell threads, as in the green and plasts. The layers below' have progressively fewer of these, the central cells red forms. The lateral union of the branches to form a solid being quite colourless. G, Section showing thick-walled cells of cortex in Brown Alga (sea- thallus is not, however, so common, nor is it carried to so high a v'eed). Simple pits (p.) enable conduction to take place readily from one to pitch of elaboration as in the Rhodophycese. In a few of the lower forms (Sphacelariacese), and in the higher forms which possess a another. H, Two adjacent cells (leptoids)of a food-conducting strand in Fucus(a Brown solid thallus often of very large size, the plant-body is no longer seaweed). The wall between them is perforated, giving passage to coarse strands formed entirely of branched cell threads, but consists of what is of protoplasm. I, End of hydroid of the thalloid Liverwort Pallavicinia, showing the thick called a true parenchymatous tissue, a solid mass of cells, that is lignified wall penetrated by simple pits. to say, formed by cell division in all directions of space. In the J, End of hydroid of the Moss Mniwm, showing particularly thin oblique Laminariacese this tissue is formed by cell division at what is end-wall. No pits. K, Optical section of two adjacent leptoids of the Moss Polytrichum juni- called an intercalary growing point, i.e., a meristematic (cellperinum. The leptoids are living and nucleated. They bulge in the neighbour- dividing) region occupying the whole of a certain zone of the hood of the very thin cross-wall. Note resemblance to H and R. thallus, and cutting off new cells to add to the permanent tissue L, Optical section of cell of conducting parenchyma in the same moss. on both sides. In the Fucacese, on the other hand, there is a Embedded in the protoplasm are a number of starch grains. M, Part of elongated stereid of a Moss. Note thick walls and oblique slit- single prismatic apical cell situated at the bottom of a groove at like pits, with opposite inclination on the two sides of the cell. growing apex of the thallus, which cuts off cells from its N, One side of end of hydroid (tracheid) of Pteridophyte, with scalariform the sides to add to the peripheral, and from its base to add to pits. O, Optical section of two adjacent leptoids (sieve-tube segments) of Pteri- the central permanent cells. The whole of the tissue of the dophyte, with sieve plates (s. pi.) on oblique end-wall and side-walls. plant is formed by the division of this apical cell. In whatP, Part of spiral hydroid (tracheid) of Phanerogam. ever. way the tissues are originally formed, however, the main Q, Three segments of a “ pitted ” vessel of Phanerogam. R, Optical section of leptoid (sieve-tube segment) of Phanerogam, with two features of their differentiation are the same. According to a law which applies also to the green and red forms, the superficial proteid (companion) cells. ‘s. pi., sieve-plate. S, Optical section of part of thick-walled stereid of Phanerogam, with cells are packed with chromatophores and form the assimilating almost obliterated cavity and narrow slit-like oblique pits. T, Part of vertical section through blade of typical leaf of Phanerogam. tissue. In these brown types with bodies of considerable thicku.e., Upper epidermal cells, with (c) cuticle, (p.) assimilating (palisade) cells. ness (Laminariaceae and Fucacese) there is, however, a further sp., Assimilating'(spongy) cells, with large lacunae, l.e., Lower epidermis, with differentiation of the internal tissues. The cells immediately st., stoma. subjacent to the superficial assimilating layer form a colourless, U, Absorbing cell, with process (root-hair) from piliferous layer of root of or nearly colourless, parenchymatous cortex, which acts as a Phanerogam. V Endodermal cell of Phanerogam, with suberized central band on radial food storage tissue, and surrounds a central medulla of elongated and transverse walls. conducting cells. The latter are often swollen at the ends, so 408

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