THE CELL-WALL. 



33 



the tissue (annular vessels in the fibro-vascular bundle of Equiseta, Zta Mais, &c.); but the 

 thickenings formed like spiral bands may often be drawn out to considerable length as 

 isolated fibres (very striking examples of these so-called untwisting spiral vessels are found 

 in the rachis of the inflorescence of Richius communis and in the leaves of Agapanthus). 

 If the thickening of the cell-wall takes place over more extended portions of the surface, 

 and if only smaller portions remain thin, these latter appear as pits of very various 

 outline, either roundish or like fissures, or, when the thickening of the cell-wall is 

 very considerable, as channels, which perforate them. These kinds of thickening most 

 frequently project on the inner side of the cell-wall ; the channels therefore run from the 



Fir,. iS.— Cell -forms of Marchaiilia foly- 

 7norpha with tliickeninjTS projecting inwards; 

 A an elater (Schleuderzelle) (one-half) from the 

 sporangium, with two spiral bands ; A' a portion 

 more strongly magnified ; B a parenchyma-cell 

 from the centre of the thallus, with thickenings 

 projecting inwards in a reticulate manner ; C a 

 thin root-hair with thickenings projecting inwards, 

 these are arranged on a spiral constriction of 

 the cell-wall ; at Z> a thicker root-hair, with pro- 

 jections thicker and branched, and spiral ar- 

 rangement still clearer. 



L- 



rh'l 



i 



"^ifi^ 



Fig. 19.—^ a young pollen-cell of Funkia 

 ovata; the knob-like thickenings projecting out- 

 wardly are still small ; in the older pollen-cell C 

 they are larger; they are arranged in lines 

 united into a net-work.. 



Fig. 20.— Ripe pollen -grain of Cichormvi 

 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 di's.— Piece of an annular vessel from the fibro-vascular bundle of the stem of Zea Mats (X550). 

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

 seen, r r the annular thickenmgs of the wall of the vessel ; y the inner substance of one of the rings 

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

 the cell. 



cavity of the cell outwards, and are there closed by a thin membrane^; when the cell 

 loses its protoplasm and dies, the latter is in many cases destroyed, and the pit or the 

 channel then becomes open (as, for instance, in Sphagnum and many wood-cells). The 

 pits, especially in elongated cells, appear to be generally arranged in spiral rows, but in 

 other cases are peculiarly grouped (Fig. 21, A). A remarkably striking form of this 

 grouping is the Sieve-structure which occurs in the sieve-cells of the fibro-vascular 

 bundles of vascular plants, generally in the septa, but also in the longitudinal walls. In 



^ Sometimes strongly thickened cell-walls with branched pit-channels show a very complicated 

 structure, e. g. in the hard testa of Bertholletia. (Cf. Millardet in Ann. des Sciences Nat., fifth series, 

 vol. vi. part 5.) 



