THE CELL-WALL. 



25 



the simplest case the thin places (pits) are densely crowded, only separated by thicker 

 ridges, and polygonal (Fig. 23, 24, si) ; they very often appear as sharply circumscribed 

 groups of numerous points ; the whole surface of such a group may then be thinner 

 than the rest of the cell-wall. But in many cases the thin part of such a pit becomes 

 absorbed, and the protoplasmic contents of adjoining cells enters into communication 

 through these narrow channels. (Fig. 88.) Sometimes the structure of these sieve- 

 plates {e.g. in Cucitrbita Pepo) becomes, when old, very peculiar and complicated from 

 further thickening and swelling of the thickened portions \ 



Fig. 25. — Pinus syl-oestris ; radial longitudinal sec- 

 tion through the wood of a rapidly growing branch ; 

 cb cambial wood-cells ; a—e older wood-cells ; t ^ t" 

 bordered pits of the wood-cells, increasing in age ; 

 st large pits where cells of the medullary rays lie next 

 the wood-cells (X550). 





Fig. 26.—/'u/iis sylvcstris; .1 transverse section of mature wood- 

 cells (XSoo) ; ni central layer of the common wall ; i inner layer, cloth- 

 ing the cavity ; z intermediate layer of the cell-wall ; t a mature 

 pit cut through the middle ; t' the same, but at a thicker part of 

 the section, the part of the cavity of the pit lying beneath is seen 

 in perspective ; t" a pit cut through beneath its inner opening ; 

 B transverse section through the cambium (x8oo); c cambium; 

 h wood-cells still young ; between them two very young wood-cells 

 with the formation of pits beginning/ t; C—/=" diagrams. 



One form of the internally projecting thicknesses which is of extremely common 

 occurrence in wood-cells and vessels, viz. the formation of Bordered Pits, deserves a 

 fuller exposition at this place. 



The formation of Bordered Pits arises thus : comparatively large spaces remain thin 

 at the commencement of the thickening of the cell-wall (Fig. 25, t] Fig. 26, B^t); and 



' Compare Niigeli, Ueber die Siebrohren von Cucurbita, in the Sitzungsbeiichte der k. bayerischen 

 Akad. der Wissenschaften. Miinchen 187 1 ; and Hanstein, Die Milchsaftgefasse. Berlin 1864. 



2 The development of these was first accurately recognised by Schacht, De maculis in plantarnm 

 vasis, &c. Bonn i86o. 



