FORMATION OF CELLS. 7 



however assist the whole by continually giving rise to new cells by division, which then, 

 on their part, undergo a further development. Such cells, which serve exclusively for 

 the purpose of producing new ones, are found at the extremities of all roots and branches, 

 and between the bark and wood of exogenous trees and shrubs. The cells produced in 

 these positions undergo a different development according to their situation, and usually 

 in such a way that aggregations of them into layers or strands follow simultaneously 

 the same mode of development. Some grow quickly in all directions, their wall remains 

 thin, the great bulk of their protoplasm becomes transformed into chlorophyll-granules, 

 they are rich in cell-sap, and serve, as we shall see hereafter, for assimilation, i. e. the 

 production of new organic substance which is formed out of the elements of the absorbed 

 nutrient material. In other parts of the same plants the cells extend greatly in length, 

 their diameter remains small, they form no chlorophyll ; a certain number remain suc- 

 culent and serve to convey assimilated substances ; other cells of the same strand thicken 

 their walls rapidly in many ways, their septa become absorbed, numerous cells in the 

 same row combine into a long tube (vessel), from which the protoplasm and the cell-sap 

 disappear; they serve then as organs of conduction for the plant. In their neighbour- 

 hood are formed the wood-cells ; these are mostly prosenchymatous, extended in length, 

 their wall greatly thickened, and its substance chemically changed (lignified) ; they form 

 collectively a firm frame-work which supports the remaining tissues, lends firmness and 

 elasticity to the whole, and is especially adapted for the rapid conduction of water 

 . through the plant. In the tissue of tubers, bulbs, and seeds most of the cells remain 

 thin-walled ; they become filled in the interior with albuminous substances, starch, 

 oil, inulin, &c., which afterwards, when new organs are being formed, serve as material 

 for the construction of new cells. In the same manner a considerable series of other 

 forms of tissue could be named, cork, the testa of seeds, the stone of stone-fruit, &c., 

 which all alike attain their needful firmness and strength by a peculiar development of 

 their cell-walls, in order to serve as protective envelopes for other masses of cells which 

 are still capable of further development ; their contents disappear as soon as the cell- 

 wall has assumed these properties, and their purpose has thus been fulfilled. 



Each of the forms of cell just spoken of, occurring in the same plant, serves 

 principally or even exclusively for one purpose only ; in correlation with this, either 

 the cell-wall, the protoplasm, the chlorophyll-granules, the cell-sap, or its granular 

 deposits, is specially developed. Very commonly these specialised cells lose the power 

 of reproduction and of multiplying by division; when they have fulfilled their function, 

 they disappear, or their lignified cell-vi^all alone remains. The whole plant, of which 

 these cells form a part, continues no less to live, since at special points it possesses cells, 

 which, at the proper time, again produce new masses of cells capable of fulfilling in their 

 turn the same functions. 



Sect. 3. Formation of Cells ^ — The formation of a new cell always 

 commences with the re-arrangement of a protoplasm-mass round a new centre ; 

 the material required is always afforded by protoplasm already present, and the 



^ H, von Mohl, Vermischte Schriften botanischen Inhalts Tubingen 1845, pp. 67, 84, 363 

 [Anatomy and Physiology of the Vegetable Cell, translated by Henfrey, London 1852]. — Schleiden 

 in Miiller's Archiv, 1838, p. 137 [Taylor's Scient. Mem., vol. II. pp. 281-312, and Sydenham 

 Society, 1847].— Unger, Bot, Zeit. 1844, p. 489; H. v. Mohl, Bot. Zeit. 1844, p. 273.— Nageli, 

 Zeitschrift fiir wiss. Bolanik, vol. I, 1844, P- 34j ^o\s. Ill, IV, 1846, p. 50. — A. Braun, Verjiingung in 

 der Natur, Freiburg 1850, p. 129 et seq. [Ray Soc. Botanical and Physiological Memoirs, 1853]. — 

 Hofmeister, Vergleichende Untersuchungen iiber die Embryobildung der Kryptog. u. Conif, Leipzig 

 1851 [Ray Soc. 1862]. — De Bary, Untersuchungen liber die Familie der Conjugaten, Leipzig 1858 — 

 Nageli, Pflanzenphys. Untersuch. Heft I. — Pringsheim, Jabrb. fur wiss. Botanik, vol. I, 1858, pp. i, 

 284, vol. II. p. I. — Hofmeister, Lehre von der Pflanzenzelle, Leipzig 1867. [Strasburger, Ueber 

 Ztllbildung und Zelllheilung, Jena iSSo. — id., Studien iiber Protoplasma, Jena 187O.] 



