THE CELL-WALL, 
21 
instance, the outer layers become woody and the inner mucilaginous, as in the 
wood-cells of the root of F/iaseoIus. 
Besides these changes in the substance of the cell-wall, which are not inifre- 
quently correlated with pecuHar 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 Snrface-gro<wth 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 4t 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 Althcea). 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 gro<voth of the cell-^all. Apical growth takes 
place when the surface-growth attains a maximum at any one spot (by interposition 
of new micellae of cellulose), while its intensity decreases in all directions from it, 
and at a definite distance reaches a minimum, so that this portion of the cell-wall 
projects 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 
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 
a 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 
surfaces. Intercalary growth of the cell- wall occurs in a typical form when the de- 
position 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 
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, nv) 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, mJ) formed by the extension of the 
annulus <w. 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. 
