THE CELL- WALL. 



19 



(p. 17) does not justify this conclusion, in so far as it is only during the folding-in of 

 the protoplasm - sac that the two new nuclei slowly separate from one another; whe- 

 ther they are formed afresh after the absorption of the mother-nucleus, or arise from 

 its division, is still uncertain. According to the more recent researches of Hanstein', 

 the bipartition of the parenchyma-cells of the pith of Dicotyledons {e. g. Sambucus, 

 Helianthus, Lysimachia, Polygonum, Silene) really precedes the division of the mother- 

 nucleus ; a mass of protoplasm, enclosing the latter, places itself in the centre of the 

 mother-cell. Even before the cell-division, two nucleoli at least are to be detected in 

 the nucleus, and soon afterwards a fine line divides the nucleus into two halves ; ' directly 

 afterwards, or at the same time, the whole layer of plasma which surrounds it shows a 

 free intersecting division surface, in which the new wall of cellulose then gradually arises.' 

 The nuclei of the two sister-cells thus lie, immediately after their production, on the 

 new division-wall : but they usually soon leave this situation ; very commonly they move 

 in opposite directions along the wall, until they arrive at spots over against those at 

 which they arose, and there they come to (temporary) rest on the older septa. Since 

 these parenchyma-cells usually divide in regular succession, two newly-formed nuclei of 

 different origin thus lie opposite to one another on each side of all the older septa. 



Whether these processes also take place in the primary parenchyma of the same 

 plants, and whether possibly they occur in all plants the cells of which are united into 

 tissues, Hanstein has not yet definitely stated. 



Sfxt. 4. The Cell-Wall ^ — The substance of the cell-wall is secreted from 

 the protoplasm. In what form it is contained in the protoplasm immediately be- 

 fore the secretion is not yet certainly known ; it always appears as a solution, and 

 then becomes first organised on the surface into a thin membrane. The substance 

 capable of forming cell-wall always consists of a combination of water, cellulose, 

 and incombustible materials (ash-constituents), but may afterwards undergo further 

 chemical changes. 



By the continual secretion of substance which forms cell-wall out of the proto- 

 plasm, and the deposition of this between the molecules of the membrane already 

 formed, this latter grows in such a manner that on one hand its surface, and on 

 the other hand its thickness, increases. The mode of both processes of growth is 

 dependent on the specific nature of the cell, and on the function which it has to 

 fulfil in the life of the plant ; it therefore varies almost infinitely. Generally the 

 growth in surface first preponderates, afterwards that in thickness. Neither the one 

 nor the other is uniform over all points of a cell-wall ; hence each cell, during its 

 growth, also changes its form ; moreover the growth of a cell-wall continues only 

 so long as it is in immediate contact on its inner side with the protoplasm. 



The want of uniformity of the growth in surface at different points causes cells 

 which are at first, for example, spherical, ovoid, or polyhedral, to become subse- 

 quently cylindrical, conical, bag-shaped, tabular, bounded by waved surfaces, &c. 

 The want of uniformity of the growth in thickness usually brings about sculpture 

 of the surface, which is very characteristic. The thickened parts may project either 



' Sitzungsbeiichte der niederrheinischen Gesellschaft in Bonn, Dec. 19, 1870, p. 230. 



2 H. von Mohl. Vermischte Schriften hot. Inhalts. Tiibingen 1845 (numerous treatises).— 

 Schacht, Lehrbuch der Anat. und Phys. der Gewachse, 1856.— Niigeli, Sitzungsbeiichte der Miinch. 

 Akademie, 1864, May and July.— Hofmeister, Die Lehre von der Pflanzenzelle, Leipzig 1867. 

 Also numerous treatises in the Botanische Zeitung. 



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