THE CELL' WALL. 



n 



One form of internal thickening which is extremely common in wood-cells and 

 vessels, viz. 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, /; Fig. 24, B, t)\ 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 sylvestris 

 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 Marchantia polymor- 

 ^A« with thickenings projecting inwards; A half 

 of an elater from the sporogonium, 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 

 arramgement still clearer. 



FIG. \().—B a young pollen-grain oK Fiinkia 

 ovata; the knob-like thickenfngs projecting out- 

 wards are still small ; in the older pollen-grain C 

 they are larger ; they are arranged in lines united 

 into a net-wtDi-k. 



Fig. 20. — Ripe pollen-grain of Cichoriutn 

 Intybus; the almost spherical substance of the 

 cell-wall is furnished with ridge-like thickenings 

 united into a net-work ; each of these bears 

 thickenings which project still more, in the form 

 of spines arranged like a comb. 



Fig. 18 *ej.— Piece of an annular vessel from the fibro-vascular bundle of the stem of Maize (X 550). 

 h h the thin cell-wall of the vessel, on which the boundary lines of the adjoining cells are clearly 

 seen ; r r the annular thickenings of the wajl of the vessel ; y the inner substance of one of the rings 

 laid open ; t the denser layer which extends over the inner side of the ring projecting into the cavity of 

 the cell. 



(Fig. 23, «-tf; Fig. 24, C, D). 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, jP", w), each half of this 

 pit-cavity communicating with the cell-cavity by a circular opening; When the wood- 



Akad. der Wissenschaften, Munchen 1861, On the actual perforation C>f Sieve-plates see Sachs 

 in Flora, 1863, p. 68, and Hanstein, Die Milchsaftgefasse, Berlin 1864, p. 23 et seq. 



^ The development of these was first accurately recognised by Schacht, De maculis in plantarum 

 vasis, &c., Bonn i860. 



