THE CELL- WALL. 
23 
One form of internal thickening which is extremely common in wood-cells and 
vessels, 'vi%. the formation of Bordered Pits'^, deserves a fuller exposition. 
The formation of Bordered Pits takes place as follows. When the cell-wall begins 
to thicken, comparatively large spaces remain thin (Fig. 23, t; Fig. 24, B, ; but, as 
the thickening augments, it reaches even the thin spaces of the wall (Fig. 23, a-e; Fig. 
24, C-F). The outline of the thin spaces of the wall in the wood of Pinus syl'vestris 
appears circular on a front view ; the edge of the thickening-mass which arches over 
it grows also in a circular manner, and gradually contracts the opening; thus the front 
view of such a pit shows two concentric circles, the larger of which corresponds to the 
original thin space (Fig. 23, /), and the smaller to the inner edge of the thickening 
Fig. 18.— Cell-forms of Maj-chantia folyino^'- 
fha with thickenings projecting- inwards; A half 
of an elater from the sporogoniiim, with two spiral 
bands ; A' a portion more strongly magnified ; 
B a parenchymatous cell from the centre of the 
thallus, with reticulate thickenings projecting 
inwards ; C a slender rhizoid with thickenings 
projecting inwards, arranged on a spiral con- 
striction of the cell-wall ; D a thicker rhizoid, 
with thicker branched projections, and the spiral 
arrangement still clearer. 
FIG. 19.— 5 a young pollen-grain Funktet 
ovata; the knob-like thickenings projecting out- 
wards are still small ; in the older pollen-grain C 
they are larger ; they are arranged in lines united 
into a net-work. 
Fig. 20.— Ripe pollen-grain of Cichoi-ium 
hitybiis; the almost spherical substance of the 
cell-wall is furnished with ridge-like thickenings 
united into ä net-work ; each of these bears 
thickenings which project still more, in the form 
of spines arranged like a comb. 
Fig. 18 ^zj.— Piece of an annular vessel from the fibro-vascular bundle of the stem of Maize (X 550). 
hh the thin cell-wall of the vessel, on which the boundary lines of the adjoiYiing cells are clearly 
seen ; r r the annular thickenings of the wall of the vessel ; y the inner substance of one of the rings 
laid open ; z"the denser layer which extends over the inner side of the ring projecting into the cavity of 
the cell. 
(Fig. 23, fl-e; Fig. 24, C, i)). Now since this process takes place on both sides of 
a partition-wall of two cells, a lenticular space is enclosed by the two overarchings, 
divided in the middle by the original thin cell-wall (Fig. 24, cw), each half of this 
pit-cavity communicating with the cell-cavity by a circular opening. When the wood- 
Akad. der Wissenschaften, München 1861. On the actual perforation of sieve-plates see Sachs 
in Flora, 1863, p. 68, and Hanstein, Die Milchsaftgefässe, Berlin 1864, p. 2^ et seq. 
^ The development of these was first accurately recognised by Schacht, De maculis in plantarum 
vasis, &c., Bonn 1S60. 
