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 m.asses 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-wall 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. vori Mohl, Vermischte Schriften botanischen Inhalts Tübingen 1845, pp. 67, 84, 362 
[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.— Nägeli, 
Zeitschrift für wiss. Botanik, vol. I, 1844, P- 34? "^'ols. III, IV, 1846, p. 50. — A. Braun, Verjüngung in 
der Natur, Freiburg 1850, p. 129 et seq. [Ray Soc. Botanical and Physiological Memoirs, 1853]. — 
Hofmeister, Vergleichende Untersuchungen über die Embryobildung der Kryptog. u. Conif., Leipzig 
1851 [Ray Soc. 1862]. — De Bary, Untersuchungen über die Familie der Conjugaten, Leipzig 1858 — 
Nägeli, Pflanzenphys. Untersuch. Heft I. — Pringsheim, Jabrb. für wiss. Botanik, vol. I, 1858, pp. i, 
284, vol. II. p. I. — Hofineister, Lehre von der Pflanzenzelle, Leipzig 1867. [Strasburger, Ueber 
Zellbildung und Zelltheilung, Jena iSSo. — id., Studien über Protoplasma, Jena 187'').] 
