22 



MORPHOLOGY OF THE CELL. 



this is repeated many times, the. appearance is presented which is figured at J, c (the 

 formation of a so-called cr.p ^). 



(b) The Groivtb in Thickness of a Cell-ivall is usually strictly localised, so that the 

 thicker parts appear as very abrupt projections on the thinner parts of the cell-wall, 

 either outside or inside. The general effect produced by the sculpture depends on 

 whether the thick or thin portions occupy the greater extent of surface. If the thicken- 

 ing is strongest at certain points only, it takes the form of knobs or spines projecting 

 outwardly (Fig. 19) or inwardly (Fig. 18, C,D)\ if it occurs most strongly in linear 

 or ribbon-shaped portions of the cell-wall, projecting ridges or bands are formed 

 on the inner or outer side. These ridge-like projections may form reticulate figures 

 (Fig. 18, B, Fig. 20, /), or rings, or spiral bands, an appearance especially frequent in 

 certain thickened tissue-cells. If the rings or spiral bands 

 which project inwards are thick and firm, and the inter- 

 mediate portions of cell-wall thin and easily destructible, 

 these thickenings may become detached even within the 

 plant, and remain lying as isolated threads of cellulose in 

 channels of the tissue, as in the annular vessels in the fibro- 

 vascular bundles of Equisetaceae, Maize, &c. ; but the spiral 

 thickenings may often also be drawn out to a considerable 

 length as isolated fibres. Very striking examples are found 

 in the rachis of the inflorescence of Ricinus communis and 

 in the leaves of Agapanthus. If the thickening aff'ects the 

 whole surface of the cell-wall more completely, the small 

 portions which remain thin will appear as Pits of very 

 various outline, either roundish or fissure-like, or, when 

 the thickening of the cell-wall is very considerable, as 

 Canals^ which perforate them. Thickenings of this kind 

 most frequently affect the inner side of the cell-wall ; the 

 canals therefore run from the cavity of the cell outwards, 

 and are there closed by a thin membrane^. When the 

 cell loses its protoplasm and dies, this membrane is in 

 many cases destroyed, and the pit or the canal then be- 

 comes open, as, for instance, in Sphagnum and in many 

 wood-cells. The pits, especially in elongated cells, are 

 generally arranged in spiral rows ; but in other cases they 

 are peculiarly grouped (Fig. 21, A). A remarkably striking 

 form is the Sie've-like or Lattice-like marking which occurs 

 in the Sie've-tubes of fibro-vascular bundles, generally in the 

 septa, but also in the side-walls. In the simplest case the 

 thin spots (pits) are densely crowded, only separated by 

 thicker ridges, and polygonal in shape ; they very often 

 appear as sharply circumscribed groups of numerous dots. 

 In many cases the thin parts of such an area become absorbed, and the protoplasmic 

 contents of adjoining cells enter into communTcation through these narrow channels. 

 Sometimes the structure of these Sieue-plates {e.g. in Cucurbita Pepo) becomes, when 

 old, very peculiar and complicated, from further thickening and swelling of the thickened 

 portions ^. 



Fig. 17. — Intercalary surface-growth of 

 CEdogonium, 



^ For further details of these somewhat complicated processes see Pringsheim, Jahrbuch fiir 

 wissen. Bot. vol. I; Hofmeister, Handbuch der phys. Bot. vol. I. p. 154; and Nageli und Schwen- 

 dener, Das Mikroskop, vol. II. p. 549. 



^ Sometimes strongly thickened cell-walls with branched pit-canals have a very complicated 

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

 vol. VI.) 



3 Compare Nageli, Ueber die Siebrohren von Cucurbita, in the Sitzungsberichte der k. bayerischen 



