THE CELL-WALL. 



ZI 



plasticity, and easy permeability to water without any considerable swelling. The 

 conversion into mucilage at length causes the cell-wall to become capable of ab- 

 sorbing great quantities of water, so as to increase its volume to a corresponding 

 extent, and to assume a gelatinous consistence. In the dry state such cell-walls 

 are hard, brittle, or flexible like horn (as the cell-walls of many Algse, the so-called 

 intercellular substance of the endosperm of Ceratonia Siliqua, of linseed, and quince- 

 mucilage). Several of these changes may occur simultaneously in a cell-wall, so 

 that, for instance, the outer layers become woody and the inner mucilaginous {e.g. 

 wood-cells of the root of Phaseolus). 



Besides these changes in the substance of the cell-wall, which are not unfre- 

 quently correlated with peculiar colourings, changes in its chemico-physical behaviour 

 may also be induced by the interposition between its molecules of considerable 

 quantities of incombustible substances, especially hme and silica. If the deposition 

 of these substances take place in sufficient quantities, they remain behind, after 

 destruction of the organic groundwork of the cell-wall, in the form of what is 

 termed an ash-skeleton. 



(a) The Surface-gro^vth causes not only an increase of the size of the cell, but also 

 changes of form, in proportion as it is wanting in uniformity at different parts of 

 the circumference ; hence cells 

 of originally dissimilar form 

 may become similar by un- 

 equal growth ; but it is much 

 more common for cells origi- 

 nally alike in form to become 

 entirely unlike. This is most 

 usually the case with the Hiulti- 

 cellular organs of the higher 

 plants, leaves, stems, and roots; 

 cells in their infancy can here 

 often scarcely be distinguished 

 from one another ; whereas in 

 the completely developed or- 

 gan the most various forms are 

 contiguous (Fig. 1 6). It is only 

 rarely, as in the growth of 

 some spores and pollen-grains, 

 that the surface-growth is so 

 uniform that the original form 



is nearly retained even after considerable increase in volume {e. g. pollen of Cucurbita 

 and Althaea). But even in these cases the uniformity is only temporary, for the pollen- 

 grains subsequently emit their pollen-tubes or the spores germinate, in both cases by the 

 local growth of their inner layer of cell-wall. This also shows at the same time that the 

 surface-growth of a cell-wall may be very different at different times ; and this indeed is 

 usually the case. From the infinite variety of the surface-growth of cell-walls, it is 

 convenient, for the sake of arrangement, to reduce the different cases to classes, 

 and to bestow names upon them'. Thus it is usual to distinguish between inter- 



Fig. i6. — From the transve 



section of a leaf of Camellia japo)iica; P paren- 

 chyma-cells with grains of chlorophyll and drops of oil ; /•" a very thin tlbro- 

 vascular bundle; v v z. large, branched, thick-walled cell, which intrudes its arms 

 between the parenchyma-cells. 



^ A good classification of the processes of growth is, of course, still more important for the 

 study of the mechanics of growth; but little has, however, yet been done in this duection, and we 

 can only give a brief abstract. 



