44 THE CELL 



structures are adorned, subserve a mechanical function by producing a 

 l'ouo'h surface and thus facilitating adhesion to the substratum which 

 in the case of pollen-grains is, of course, represented by the stigmatic 

 surface. Centrifugally produced thickenings, of uncertain or entirely 

 obscure significance, occur on the outer surface of epidermal cells and 

 hairs, where they usually consist of slender ridges or minute nodules, 

 less frequently taking the form of large prominent knobs. 



Centripetal thickenings display far greater diversity as regards both 

 their qualitative and their quantitative development. For various 

 reasons a portion of the cell-wall almost always remains thin, in 

 connection with this type of thickening. Sometimes partial thickening 

 suffices to provide the required degree of mechanical strength and 

 rigidity ; thus, in the case of a velamen-cell or in that of a reticulate 

 wood-vessel, the absence of thickening over a considerable area merely 

 represents an economy of material. In other instances it is important 

 that the growth in thickness of the membrane, which must take place 

 in order to satisfy mechanical requirements, should not render the cell 

 or vessel incapable of altering its shape, of increasing or diminishing its 

 volume, and, if need lie, of growing in length. A compromise is there- 

 fore effected, whereby certain regions of the wall are maintained in an 

 unthickened, pliant and extensible condition. In the case of the water- 

 tissue of Aesckynanthus, for example, the presence of annular thickening 

 fibres in the walls does not in the least hinder the cells from contract- 

 ing when water is withdrawn from them. Similarly the differentiation 

 of annular and spiral wood-vessels does not prevent subsequent elonga- 

 tion of a stem. The same principle is illustrated by the unicellular 

 hydathodes of Gonocaryum (Ch. X. II. A) ; here the funnel-shaped upper 

 portion has a very thick wall, but the unthickened condition of the 

 lower portion allows the volume of the entire hydathode to change in 

 accordance with variations of turgor. 



Most frequently, however, the retention of permanently un- 

 thickened areas in a cell-wall serves primarily to facilitate diffusion 

 between adjacent cells. For although thickened walls are by no 

 means impervious to dissolved substances, yet the rate of diffusion 

 through a membrane varies inversely as the thickness of the 

 latter ; consequently the presence of unthickened areas in cell-walls 

 must be highly advantageous from the point of view of translocation. 

 These readily permeable spots generally take the shape of sharply 

 defined areas of approximately circular cross-section, known as pits. In 

 accordance with their function, the pits on the two sides of a cell-wall 

 always correspond exactly, so that the adjoining pit-cavities are separated 

 only by the thin primary closing-membrane. The ectoplast lining the 

 pit-cavity adheres closely to the closing-membrane, and in this region 



