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 (Rivularieae 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 Nageli 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-^all into Systems of layers {Shells) ^vith 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 ; /. 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 Algae, 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, 
