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Bs | ellipsoidal (Fig. war hour-glass-shaped (Fig. 6); stellate 
(Figs. 7 and 11); disc-shaped (Fig. 8); crescent-shaped 
~ 
(Fig. 9); tabular (Fig. 10); tetrahedral (Fig. 12); prismatic 
(Fig. 13); fusiform (Figs. 14, 15); polyhedral (Fig. 16); 
irregularly branched (Fig. 17). 
All these forms, to which many others might be added, are mateo 
“from cells which are originally more or less spherical, according as the 
pressure to which they are subject from the adjacent cells is greater or 
less, and according as the vigour of their growth on all sides is more or 
less uniform. It is by far most probable that the growth of the cell- 
wall, when once formed, takes place solely by zztussusception, that is, by~ 
the intercalation of fresh particles of cellulose among those already in 
‘existence. This process is the essential element of those long- enduring _ 
changes of a physical and chemical character to which the cell-wall is 
subject during its growth, causing it to be of essentially different — 
nature from the shell of a mussel, on the inner side of which new 
layers are being constantly deposited. The wall serves not only for the - 
external protection of the cell, but is most intimately connected with its 
life ; so that even when the cell has long ceased to exercise an active 
influence on the life of the plant by the formation o fnew cells or by the 
secretion of special substances within it, other changes are still proceed- 
ing in the cell-wall, which may often be perceived even by the naked 
eye ; as in ebony-wood, the innermost layers or ‘duramen’ of which — 
- become of a darker colour than the ‘alburnum’ or layers’ of wood 
which lie nearest to the bark. 
The cell-wall appears at first as a thin, apparently. | 
structureless membrane (the primary cell-wall); in the 
course of its growth, however, several layers, c¢hzckening- 
layers, are apparently deposited at an early period on the 
inside. Although under the microscope these layers seem 
very clearly defined from one another and from the primary 
cell-wall, such a definition does not actually always exist 
(see Figs. 19-21). The fact that the cell-wall appears to 
consist of several layers depends on the unequal density 
of the separate lamellz ; and this can be explained by an 
unequal absorption of ones the layers appearing as if 
quite unstratified when dry. Always less evident than the 
thickening-layers, there can be perceived on the cell-wall 
The Cell as an fadtoidudl ; 15 
