THE CELL- WALL. 21 



instance, the outer layers become woody and the inner mucilaginous, as in the 

 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 

 also arise from the interposition between its micellae of considerable quantities 

 of incombustible substances, especially lime and silica. If the deposition of these 

 substances take place in sufficient quantities, they remain behind, after the combus- 

 tion of the organic groundwork of the cell-wall, as an ash-skeleton. 



(a) The Surface^grcfivth causes not only an increase in the size of the cell, but also 

 changes in its form, when it takes place irregularly at different points of the circum- 

 ference. Hence cells of originally dissimilar form may become similar by unequal 

 growth; but it is much more common for cells originally alike in form to become 

 entirely unlike. This is most usually the case in multicellular organs of the higher 

 plants, such as leaves, stems, and roots; in their earliest state their cells are often 

 scarcely distinguishable from one another ; whereas in the completely developed organ 

 the most various forms are juxtaposed (Fig. i6). 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 size {e.g. pollen of 

 Cucurbita and Althad). 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 the inner layer of the 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 intercalary and apical growth of the cell-wall. Apical growth takes 

 place when the surface-growth attains a maximum at any one spot (by interposition 

 )f new micellae of cellulose), while its intensity decreases in all directions from it, 

 md at a definite distance reaches a minimum, so that this portion of the cell-wall 

 )rojects as a point, or appears as the rounded apex of an excrescence or of a cylindrical 

 [tube, as in hairs or filamentous Algae. If several points of apical growth occur in a cell 

 I which was originally round, it may become star-shaped ; if new points of growth are 

 formed behind the continuously growing end of a tubular cell, it branches, as in many 

 [filamentous Algae, hyphae of Fungi, Faucheria, Bryopsis. Hofmeister^ distinguishes as 

 peculiar form of apical growth the case in which the maximum of growth is localised 

 in a line instead of at a point ; this may occur as the line of intersection of two curved 

 irfaces. Intercalary growth of the cell- wall occurs in a typical form when the de- 

 )osition of new substance is localised in a zone of the cell-wall; this zone extends, 

 [and a fresh piece of cell-wall is by degrees intercalated between the old ones. Very 

 similar to this is the common occurrence of growth in the whole of the side-wall of 

 a cubical, tabular, or cylindrical cell, as, for example, in the cells of Spirogyra, and the 

 I parenchymatous cells of growing roots and stems of Phanerogams (see Fig. i). (Edo- 

 ^gonium presents a peculiar case of intercalary surface-growth (Fig. 17). Below a 

 septum an annular deposit of cellulose {A, iv) is formed ; at this place the cell-wall 

 splits, as if by a circular cut, into two pieces, and these separate from one another, 

 but remain united by a zone of cell-wall {B, iv) formed by the extension of the 

 annulus ou. After the intercalation of this new zone, cell-division follows ; and, since 



^ 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 direction, and we 

 can only give a brief abstract. 



* Handbuch der physiol. Botanik, vol. I. p. 162. 



