26 



MORPHOLOGY OF THE CELL. 



after increased thickening, the thickening-mass which is always projecting inwardly, 

 acquires a larger surface, and forms an arch over the thin part of the wall (Fig. 25, a~e\ 

 Fig. 26, C-F). The outline of the thin parts of the wall in the wood of Pinus syl'vestris 

 appears circular on a front view ; the rim of the thickening-mass which becomes arched 

 over it grows also in a circular manner, gradually contracting the opening ; and thus the 

 front view of such a pit appears in the form of two concentric circles, the larger of which 

 represents the original dimensions of the thin parLs of the cell-wall (Fig. 25, cb, t), and 

 the inner one the gradually widening circular rim of the thickening (Fig. 25, a-e; Fig. 26, 

 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, which is divided in the middle 

 into two equal parts by the original thin lamella of the cell-wall (Fig. 26, F, nv) ; each 

 half of this pit-cavity communicates with the cell-cavity by a circular opening. If the 

 wood-cells lose their protoplasm, and become filled with air and water, this thin mem- 



FIG. ■2'!.— Dahlia variabilis ;\\2i\\ of a vessel with bordered pits 

 from the succulent root-tuber ; A front view of a piece of the 

 wall of a vessel from without ; B transverse section of the same 

 (horizontal, at right angles to the paper) ; C longitudinal section 

 (vertical, at right angles to the plane of the paper) ; q septum ; 

 a the original thin thickening-ridges; b the expanded part of 

 the thickening-masses, formed later and over-arching the pit ; 

 c the fissure through which the cavity of the pit connnunicates 

 with the cell-cavity. At a and ^ the corresponding front view 

 is appended in order to make the transverse and longitudinal 

 sections more clear (x 800). 



FIG. 28. — Dahliavariabilis, from the 

 root-tuber ; P parenchymatously deve- 

 loped wood-cells ; Fa piece of the wall 

 of a vessel, where it adjoins a paren- 

 chymatous wood-cell ; a b the thicken- 

 ing-masses of the wall of the vessel cut 

 through at right angles ; r, t fissure of 

 the pit ; d simple pits in the parenchy- 

 matous wood-cells (X800). 



brane is destroyed (as in Fig. 26, E)\ the pit-space forms a single cavity, which is en- 

 closed between the over-arching thickening-masses of the partition-wall, and is united, 

 right and left, with the adjoining cell-cavities by a circular opening (Fig. 26, A,D,E). 

 In Pinus syl'vestris the pits are large and distant from one another, and the whole pro- 

 cess may be easily traced step by step. The process appears somewhat different when 

 pits lie very near to one another, as in Pitted Vessels, In this case the thickening first 

 presents itself in the form of a net-work, which surrounds the thin parts of the cell- 

 wall in the form of roundish polygonal meshes, as may be very easily recognised in 

 young maize-roots, for instance. Fig. 27, ^, represents a portion of the side-wall of an 

 already mature vessel ^ of the root-tuber of Dahlia. The ridges which originally ap- 

 pear on the thin cell-wall are indicated by a and are left clear ; they enclose elliptical 

 meshes pointed at both ends. As the thickening continues, each ridge retains its 



On the idea of a vessel, see chap. ii. 



