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 ANATOMY

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that the cross-wall separating two successive cells has a larger surface than if the cells were of unifoi'm width along their entire length. Cells of this type are often called trumpet-hyphen, and in some genera of Laminariaceai those at the periphery of the medulla simulate the sieve-tubes of the higher plants in a striking degree, even developing the peculiar substance callose on or in the perforated cross-walls or sieve - plates. A specialized conducting tissue of this kind, used mainly for transmitting organic nitrogenous substances, is always developed in plants where the region of assimilative activity is local in the plant-body. This is the case in the Fucaceae, and in a very marked degree in the Laminariaceee in question, where the assimilative frond is borne at the end of a long supporting and conducting stipe. The tissue developed to meet the demands for conduction in such cases always shows some of the characters described. It is known as leptom, each constituent cell being a leptoid. In addition to the cell types described, it is a very common occurrence in these bulky forms for rhizoid-like branches of the cells to grow out mostly from the cells at the periphery of the medulla, and grow down between the cells, strengthening the whole tissue, as in the EhodophyceEe. This process may result in a considerable thickening of the thallus. In many Laminariacese the thallus also grows regularly in thickness by division of its surface layer, which thus forms a secondary ineristem. The simpler Fungi, like the simpler Green Algae, consist of single cells or simple or branched cell threads, but among the higher kinds a massive body is often formed, particuUem riifferenlarly ma in yconnexion with the formation of spores, and di tissue-differentiation. ex]1ipit considerable tiatioa in ^ characteristic feature of the fungal vegetative plantrung • body {mycelium) is its formation from independent ccenocytic tubes or cell threads. These branch, and may be packed or interwoven to form a very solid structure ; but each grows in length independently of the others and retains its own individuality, though its growth in those types with a definite external form is of course correlated with that of its neighbours, and is subject to the laws governing the general form of the body. Such an independent ccenocytic branch or cell thread is called a hypha. Similar modes of growth occur among the Siphoneous green and also among the red seaweeds. A solid fungal body may usually be seen to consist of separate hyphse, but in some cases these are so bent and closely interwoven that an appearance like that of ordinary parenchymatous tissue is obtained in section, the structure being called pseudoparenchyma. By the formation of numerous cross-walls the resemblance to parenchyma is increased. The surface-layer of the body in the massive Fungi differs in character according to its function, which is not constant throughout the class, as in the Algse, because of the very various conditions of life to which different Fungi are exposed. In many forms its hyphse are particularly thick-walled, and may strikingly resemble the epidermis of a vascular plant. This is especially the case in the lichens (symbiotic organisms composed of a fungal mycelium in association with algal cells), which are usually exposed. to very severe fluctuations in external conditions. The formation of a massive body naturally involves the localization of the absorptive region, and the function of absorption (which in the simpler forms is carried out by the whole of the vegetative part of the mycelium penetrating a solid or immersed in a liquid substratum) is subserved by the outgrowth of the hyphse ol the surface-layer of that region into rhizoids, which, like those of the Algae living on soil, resemble the root-hairs of the higher plants. The internal tissue of the body of the solid higher fungi, particularly the elongated stalks {stipes) of the fructifications of the Agarics, consists of hyphse running in a longitudinal direction, which no doubt serve for the conduction of plastic food substances, just as do the “ trumpet-hyphse,” similar in appearance, though not in origin, of the higher brown seaweeds. (In one genus {Lactarius) ‘ ‘ milk-tubes, ” recalling the laticiferous tubes of many vascular plants, are found.) These elongated hyphae are frequently thickwalled, and in some cases form a central strand, which serves to resist longitudinal pulling strains. This is particularly marked in certain lichens of shrubby habit. The internal tissues, either consisting of obvious hyphae or of pseudoparenchyma, may also serve as a storehouse of plastic food substances. Looking back over the progress of form and tissuedifferentiation in the Thallophyta, we find that, starting from the simplest unicellular forms with no external differentiation of the body, we can trace an increase in complexity of organization everywhere determined by the principles of the division of physiological labour and of the adaptation of the organism to the needs of its environment. In the first place there is a differentiation of fixing organs, which in forms living on a soft nutrient substratum penetrate it and become absorbing organs. Secondly, in

PLANTS

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the Algse, which build up their own food from inorganic materials, we have a differentiation of supporting axes from assimilating appendages, and as the body increases in size and becomes a solid mass of cells or interwoven threads, a corresponding differentiation of a superficial assimilative system from the deep-lying parts. In both Algse and Fungi the latter are primarily supporting and food-conducting, and in some bulky Brown Seaweeds where assimilation is strongly localized some of the deep cells are highly specialized for the latter function. In the higher forms a storage and a mechanically-strengthening system may also be developed, and in some aerial Fungi an external protective tissue. The “ hyphal ” mode of growth, i.e., the formation of the thallus, whatever its external form, by branched, continuous or septate, coenocytic tubes (Siphonese and Fungi), or by simple or branched cell threads, in both cases growing mainly or entirely at the apex of each branch, is almost universal in 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. Bryophyta. The Bryophyta, the first group of mainly terrestrial plants, exhibit considerably more advanced tissue differentiation, in response to the greater complexity in the conditions of life on land. But the lowest Hepaticai have an extremely simple vegetative structure, little more advanced than that found in some of the higher Green Algse and very much simpler than in the large red and brown seaweeds. The thallus, however, always consists of true parenchyma, and is entirely formed by the cutting off of segments from an apical cell. A sufficient description of the thallus of the Liver-worts, e.g., Fegatella, will be found in the article Muscink/E {Ency. Brit. vol. xvii.). We may note the universal occurrence on the Liverlower surface of the thallus of fixing and absorbing WOrts. rhizoids in accordance with the terrestrial life on soil {cf (Edocladium among the Green Alg;e). The Marchantiacese (see article MusciXEiE) show considerable tissue-differentiation, possessing a distinct assimilative system of cells, consisting of branched cell threads packed with chloroplasts and arising from the basal cells of large cavities in the upper part of the thallus. These cavities are completely roofed by a layer of cells ; in the centre of the roof is a pore surrounded by a ring of special cells. The whole arrangement has a strong resemblance to the lacunae, mesophyll and stomata, which form the assimilative and transpiring (water-evaporating) apparatus in the leaves of flowering plants. The frondose (thalloid) Jungermanniaceae show no such differentiation of an assimilating tissue, though the upper cells of the thallus usually have more chlorophyll than the rest. In three genera—Pallavicinia, Symphyogyna, and Hymenophyton— there are one or more strands or bundles consisting of long thickwalled fibre-like 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. II). These cells are not living in the adult state though they sometimes contain the disorganized remains of protoplasm. There is little doubt that their function is to conduct water through the thallus, the assimilating parts of which are in these forms often raised above the soil and comparatively remote from the rhizoid-bearing (waterabsorbing) region. Such differentiated water-conducting cells we call hydroids, the tissue they form hydrom. The sporogonium of the liverworts is in the simpler forms simply a spore-capsule with arrangements for the development, protection, and distribution of the spoils. As such it falls outside the scheme of this article, but in one small and peculiar group of these plants, the Anthocerotece, a distinct assimilating and transpiring system is found in the wall of the very long cylindrical capsule, clearly enabling the sporogonium to be largely independent of the supply of elaborated organic food from the thallus of the mother plant (the gametophyte). A richly chlorophyllous tissue with numerous intercellular spaces communicates with the exterior by stomata, strikingly similar to those of the vascular plants (see below). If