22 
MORPHOLOGY OF THE CELL. 
this is repeated many times, the appearance is presented which is figured at A, c (the 
formation of a so-called cap ^). 
(b) The Groqjuth in Thickness of a Cell-^all is usually strictly localised, so that the 
thicker parts appear as very abrupt projections on the thinner parts of the cell-wall, 
either outside or inside. The general effect produced by the sculpture depends on 
whether the thick or thin portions occupy the greater extent of surface. If the thicken- 
ing is strongest at certain points only, it takes the form of knobs or spines projecting 
outwardly (Fig. 19) or inwardly (Fig. 18, C, D) ; if it occurs most strongly in linear 
or ribbon-shaped portions of the cell-wall, projecting ridges or bands are formed 
on the inner or outer side. These ridge-like projections may form reticulate figures 
(Fig. 18, B, Fig. 20, /), or rings, or spiral bands, an appearance especially frequent in 
certain thickened tissue-cells. If the rings or spiral bands 
which project inwards are thick and firm, and the inter- 
mediate portions of cell-wall thin and easily destructible, 
these thickenings may become detached even within the 
plant, and remain lying as isolated threads of cellulose in 
channels of the tissue, as in the annular vessels in the fibro- 
vascular bundles of Equisetaceae, Maize, &c. ; but the spiral 
thickenings may often also be drawn out to a considerable 
length as isolated fibres. Very striking examples are found 
in the rachis of the inflorescence of Ricinus communis and 
in the leaves of Agapanthus. If the thickening aflfects the 
whole surface of the cell-wall more completely, the small 
portions which remain thin will appear as Pits of very 
various outline, either roundish or fissure-like, or, when 
the thickening of the cell-wall is very considerable, as 
Canals, which perforate them. Thickenings of this kind 
most frequently aifect the inner side of the cell-wall ; the 
canals therefore run from the cavity of the cell outwards, 
and are there closed by a thin membrane ^ When the 
cell loses its protoplasm and dies, this membrane is in 
many cases destroyed, and the pit or the canal then be- 
comes open, as, for instance, in Sphagnum and in many 
wood-cells. The pits, especially in elongated cells, are 
generally arranged in spiral rows ; but in other cases they 
are peculiarly grouped (Fig. 21, A). A remarkably striking 
form is the Sie-ve-like or Lattice-like marking which occurs 
in the Sie've-tubes of fibro-vascular bundles, generally in the 
septa, but also in the side-walls. In the simplest case the 
FIG. x7.-intercaiary surface-growth of ^hiu spots (pits) are dcuscly crowdcd, ouly Separated by 
cEdog'onmm. thicker ridges, and polygonal in shape; they very often 
appear as sharply circumscribed groups of numerous dots. 
In many cases the thin parts of such an area become absorbed, and the protoplasmic 
contents of adjoining cells enter into communication through these narrow channels. 
Sometimes the structure of these Sie've-plates {e.g. in Cucurbita Pepo) becomes, when 
old, very peculiar and complicated, from further thickening and swelling of the thickened 
portions ^ 
^ For further details of these somewhat complicated processes see Pringsheim, Jahrbuch für 
■wissen. Bot. vol. I; Hofmeister, Handbuch der phys. Bot. vol. I. p. 154; and Nägeli und Schwen- 
dener, Das Mikroskop, vol. II. p. 549. 
2 Sometimes strongly thickened cell-walls with branched pit-canals have a very complicated 
structure, e. g. in the hard testa of Bertholletia. (See Millardet in Ann. des Sciences Nat., fifth series, 
vol. VI.) 
3 Compare Nägeli, Ueber die Siebröhren von Cucurbita, in the Sitzungsberichte der k. bayerischen 
