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 CYTOLOGY

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of the spindle, where they form the centres towards which cell-wall is very rarely uniform. It is thickened more in the radiating fibres in the cytoplasm converge. The some places than in others, and thus are formed the spiral, centrosome or centrosphere is usually regarded as the annular, and other markings, as well as the pits which dynamic centre of the cell and a special organ of divi- occur on various cells and vessels. Besides the internal or sion; but its absence in many groups of plants does centripetal growth, some cell-walls are thickened on the not lend support to this view so far as plant-cells are outside, such as pollen grains, oospores of Fungi, cells concerned. of Peridinese, &c. This centrifugal growth must apparently The membrane which surrounds the protoplasts in the take place by the activity of protoplasm external to the majority of plants is typically composed of cellulose, cell. The outer protective walls of the oospores of some together with a number of other substances Fungi are formed out of protoplasm containing numerous are nown as ^membrane k poetic compounds. Some nuclei, which is at an early stage separated from the proto* of these have a neutral reaction, others react as plasm of the oosphore. In the Peridineae, Diatoms, and feeble acids. They can be distinguished by their insolu- Desmids, according to recent researches, the thickenings on bility in cuprammonia, which dissolves cellulose, and by the outer walls of the cells are due to the passage of prototheir behaviour towards stains, some of which stain pectic plasm from the interior of the cell to the outside, through substances but not cellulose. Cellulose has an affinity for pores which are found perforating the wall on all sides. Cell-walls may become modified by the impregnation of acid stains, pectic substances for basic stains. The cellmembrane may become modified by the process of lignifi- various substances. Woody or lignified cell-walls appear cation, suberization, cuticularization, or gelatinization. In to contain substances called coniferin and vanillin, in the Fungi it is usually composed of a modified form of addition to various other compounds which are imperfectly cellulose known as Fungus cellulose, which,' according to known. Lignified tissues are coloured yellow by aniline Mangin, consists of callose in combination either with sulphate or aniline chloride, violet with phloroglucin and cellulose or pectic compounds. It is formed by the proto- hydrochloric acid, and characteristic reactions are also plasm, either by the direct conversion of the outer cyto- given by mixtures containing phenol, indol, skatol, thallin plasmic layer into the cell-wall, or by the excretion of the sulphate, &c. (see Zimmerman’s Microtechnique). Staining substances which form the cell-wall. In the formation of reagents can also be used to differentiate lignified cell-walls. the new transverse walls which are produced in cell-division, Cuticularized or suberized cell-walls occur especially in the cell-plate or protoplasmic membrane produced in the those cells which perform a protective function. They are equatorial plane of the spindle-fibres appears to be trans- impervious to water and gases. Both cuticularized and formed directly into the primary cell-wall. According to suberized membranes are insoluble in cuprammonia, and Gardiner’s recent researches, the nodes of the spindle-fibres are coloured yellow or brown in a solution of chlor-iodide persist as the connecting threads of the protoplasts, and of zinc. It is probable that the corky or suberized cells are not, as was formerly supposed, merged in the substance do not contain any cellulose (Gilson, Wisselingh); whilst of the cell-wall. It is probable, therefore, that the cell- cuticularized cells are only modified in their outer layers, plate is not, as is usually stated, formed by the fusion of cellulose inner layers being still recognizable. The the thickened connecting fibres, but that it is formed out suberized and cuticularized cell-walls appear to contain a of plasmatic substances conducted to and deposited in the fatty body called suberin, and such cell-walls can be cell-plate region by them. According to Strasburger, the stained red by a solution of alcannin, the lignified and cell-plate splits into two, and the new cell-wall is formed cellulose membranes remaining unstained. as a secretion product between them. Thick cell-walls Except in the unicellular plants the cell is not an indeexhibit a distinct stratification, and the separate layers are pendent unit. Apart from their dependence in various composed of thin lamellae. The primary cell-wall persists as ways upon neighbouring cells, the protoplasts of the middle lamella, which appears to be of different chemical all plants are probably connected together by fine^0''0" composition from the rest of the wall, and to be composed strands of protoplasm which pass through ^ continuity. of pectic acid, or a salt of it. By the solution of this cell-wall (Tangl, Bussow, Gardiner, Kienitzmiddle lamella the cells can be separated from each other. Gerloff, and others) (Fig. 2). In Finns the presence of This may take place even in young cells with thin cell- connecting threads has recently been demonstrated walls. The substance contained in the intercellular spaces, throughout all the tissues of which are formed in some cases between the cells, is prob- the plant. These protoplasmic ably related to the pectic substances. In many cells strands are, except in the case there is a distinct striation visible in the cell-wall in of sieve tubes, so delicate that addition to the lamination and stratification. These striae special methods have to be run obliquely and across each other, but the two systems employed to make them visible. do not occur in the same lamella. We know very little of The basis of these methods the minute structure of the cell-wall beyond its lamination consists in causing a swelling and striation. Nageli considered it to be composed of of the cell-wall by means of minute particles of cellulose—Micella?—each surrounded sulphuric acid or zinc chloride, by water. Wiesner suggests that it consists of minute and subsequent staining with particles or dermatosomes connected together by a delicate Hoffmann’s blue or other anilprotoplasmic network; and there is a certain amount of ine dyes. Fresh tissue may evidence to show that it is composed of radiating elements be used, or tissue killed and somewhat similar to those which are supposed to occur hardened in dilute solution of in starch grains. The growth of the cell-wall takes iodine, or in osmic acid—uranplace by the addition of new layers to those already formed. ium-nitrate mixture. Alcohol Fig. 2.—Continuity of protoplasm of Tamus communis (A These layers are secreted by the protoplasm by the direct is not suitable. The results so of cells endosperm of Lilium Marapposition of substances on those already in existence; far obtained show that the con- and tagon (B). {After Gardiner.) and they may go on increasing in thickness, both by necting threads may be either apposition and by the intussusception of particles probably “ pit-threads ” which traverse the closing membrane of the carried in through the protoplasmic fibres, which penetrate pits in the cell-walls (Fig. 2, B), or “ wall-threads ” which the cell-wall as long as the cell lives. The growth of the are present in the wall of the cell (Fig. 2, A). Both pitS. III.-43