3^ 



MORPHOLOGY OF THE CELL. 



whole, increasing by intussusception ; by subsequent internal differentiation, masses 

 (shells) of layers are formed, differing in their chemical and physical properties; the 

 outer firm cuticularised shell (exospore, extine) remains unchanged, and is thrown off as 

 an envelope, while an inner mass of layers (endospore, intine) begins a new growth with 

 the germination of the spores or the development of the pollen -tubes. A similar process 

 occurs with many filamentous Algae (Rivulariese and Scytonemeae), where a large number 

 of cell-wall-layers are successively formed one within another, while from time to time 

 the older masses of layers cease to increase, and are broken through by the growing 

 filament, which now forms new cell-wall-layers (see Niigeli und Schwendener, Das 

 Mikroskop, 2nd ed., p. 547). It need scarcely be mentioned that these facts do not 

 contradict the theory of growth by intussusception, but only correspond to particular 

 modifications in the life of the cell. 



Fig. 34. — Pollen-mother-cell of Cucurbita Pepo; sg the externa common layers of the mother-cell undergoing 

 absorption; sp the so-called 'special mother-cells' consisting of masses of layers of the mother-cell-wall which surround 

 the young pollen-grains ; they also are afterwards absorbed ; ph the wall of the pollen-grain ; its spines grow outwards and 

 penetrate the 'special mother-cell;' v hemispherical deposits of cellulose on the cell-wall of the pollen-grain, from which 

 the pollen-tubes are afterwards formed; / the contracted protoplasm of the pollen-grain (the preparation was obtained 

 by section of an anther which had lain for some months in absolute alcohol) ( x 550). 



(e) Differentiation of the Cell-<zvall into Systems of layers {Shells) ivith different chemical 

 and physical properties. 



Very young and thin cell-walls, while still in rapid growth, as also many older 

 ones, are composed throughout their whole thickness of what has been termed pure 

 cellulose ; i. e. they are easily permeable by water, only slightly extensible or capable 

 of swelling, very elastic, colourless, and 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 asci of Lichens). Together with these common 

 properties, they may, according to the nature of the cell, possess many other peculiar 

 reactions. Among older fully developed cells, succulent thin-walled parenchymatous 

 cells of the higher plants behave for the most part in this manner, many thick-walled 

 cells of Algse, and — with the exception of the blue colour produced by iodine and 

 sulphuric acid, and by Schultz's solution — most hyphae of Fungi and Lichens. 



With more strongly thickened cell-walls (rarely with moderately thin ones, as some 

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

 in consequence of this the cell-wall is at once seen to consist of two or more Shells ^, each 



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

 in paragraph (d), where it implies a regularly alternating difference in the proportion of water, 



