THE CELL- WALL. 



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of cell-walls enclosed within one another are gradually formed, while from time to time 

 the older masses of layers cease growing, and are pierced by the growing filament, 

 which now forms new layers of cell-wall (cf. Niigeli und Schwendener : Das Mikroskop, 

 II. p. 551). It need scarcely be mentioned that tehse appearances do not contradict 

 the theory of the growth of the 

 cell-wall by intussusception, but 

 only represent, in general, par- 

 ticular modifications of the life 

 of the cell. 



(e) Differentiation of the Cell- 

 ivall into Systems of Layers [Shells) 

 <zuith differeJit chemical and physical 

 properties. 



Very young and thin cell-walls, 

 while still in rapid growth, as also 

 many older ones, are constituted 

 throughout their whole thick- 

 ness of what has been termed 

 pure cellulose ; /. e. they are ea- 

 sily permeable by water, only 

 slightly extensible and capable of 

 swelling, very elastic, colourless, 

 soluble in sulphuric acid ; with 

 iodine and sulphuric acid they 

 assume an intense blue colour, as 

 also with Schultz's solution, rarely 

 with solution of iodine alone (as 

 the spore-sacs of Lichens). To- 

 gether with these common pro- 

 perties, they may, according to the 

 •nature of the cell, show also many 

 peculiar reactions. Among the 

 older developed cells, the greater 

 number of succulent thin-walled 

 parenchyma - cells of the higher 

 plants behave in this manner, 

 many thick-walled cells of Algae, 

 and, — with the exception of the 

 blue colour produced by iodine 

 and sulphuric acid, and by Schultz's solution, 

 and Lichens. 



With more strongly thickened cells (rarely with moderately thin ones, e. g. some 

 cork-cells), whole masses of layers behave in a different manner chemically and physic- 

 ally, so that the cell-wall appears split into two or more shells \ each of which may again 

 exhibit numerous layers and the striation already described. In the case of free-lying 

 cells which require protection (as pollen or spores), or of those which themselves serve 

 as a protection to other tissues (as cork), an outermost shell (of greater or less thickness) 

 of each cell-wall is transformed into cork or cuticle ; w^hen the cells are destined to 



Fig. 37. — A germinating pollen-grain oi Ctccurbita Pepo, which has emitted 

 a pollen-tube (sp) into a papilla of the stigma. The cell-wall of the pollen- 

 grain consists of a cuticularised extine [e), and an intine capable of growth (z) ; 

 the latter is greatly thickened at certain places [B i) ; on each thickening-mass 

 the extine forms a roundish lid (d) ; when the pollen-grain is preparing for ger- 

 mination, the thick parts of the intine swell, and thus tilt up and lift off the 

 lid of the e.Ktine ; the pollen-tubes are formed of one or two of these thicken- 

 ing-masses (X550). 



-the greater number of filaments of Fungi 



* It is desirable to employ the expression layers (Schichten) only in the sense mentioned 

 under (d), where it imphes a regularly alternating difference in the proportion of water, as in 

 the striae ; but in that case another term must be employed for the formations now under consider- 

 ation ; the expression ' shells ' (Schalen) appears to me to answer the purpose. 



D 



