92 



LECTURE VI. 



however, a far more conspicuous coarser structure in the same cells : this is visible 

 with w^eaker magnifying powers, and makes itself chiefly evident by individual parts 

 of a cell-wall being much thinner or much thicker than accords with the common 

 thickness. We are here concerned with what may be termed the sculpture of the 

 surface. The commonest form of this sculpture consists in that, on cell-walls not 

 excessively thick, single or grouped roundish areas remain thin during the growth 

 in thickness of the whole wall, or do not take part in the growth in thickness 

 generally. We term such areas pits, and the cell-wall itself pitted. If the parts 

 of the wall lying between the pits grow very much in thickness, the pits appear 

 no longer merely as thin areas, but as canals running through the thickness 

 of the wall from the cavity of the cell, and closed exteriorly by a very thin 

 membrane. The pits lie in the partition wall between two neighbouring cells of 

 a tissue, and the pit-canals run on both sides so as to meet one another. 

 The thin membrane which closes the pit of one side forms also at the same time 

 the closure for the pit of the other side. If, therefore, we imagine the cells not 

 isolated, but in connection as usual, like the chambers of a house, and separated by 



their walls, the pits and pit-canals appear 

 like holes, through which the neighbouring 

 chambers communicate ; in such a manner, 

 however, that a very fine membrane is 

 always still present in the middle, by means 

 of which the neighbouring cell-cavities are 

 in fact separated from one another. It is 

 easy to see, and is to be insisted upon still 

 more in detail later on, that the exchange 

 of sap between neighbouring cells must be 

 facilitated by means of the pitting; especi- 

 ally when the partition-walls are of con- 

 siderable thickness, whereby of course the diffusion movements from cell to cell 

 would be rendered very difficult without the presence of numerous pits. 



Another form of sculpture of the cell-wall, also very frequent, shows itself in 

 that not thin areas, as in the pitting, but strongly thickened areas stand out 

 as peculiarly formed parts of the wall, which otherwise remains very thin. 

 Sometimes the thickened areas appear as cones projecting inwards, or they form 

 massive rings, which often become loose and fall out on cutting succulent stems 

 and leaves; yet more frequently, the thickened part of the cell-wall forms a 

 spiral band, running on the inner side of the thin wall, somewhat like a spirally- 

 wound wire fitted in a glass tube of suitable width. Very often two or three such 

 spiral bands with parallel windings are present, and, like the rings, these also often 

 become loosened on cutting or tearing off leaves and stalks, and may then be 

 perceived even with the unaided eye as threads of extreme fineness but considerable 

 length : these are very distinct for example in the leaves of Agapanihus when torn 

 across. Finally, a common form is the so-called reticulate thickening, which we 

 may suppose to be derived from the spiral by connections or anastomoses being 

 established between the parallel spiral bands, so that a mesh-work arises. If we 

 suppose the meshes of this net-work very narrow, and the thick bands relativel}- 



Fig. 93.— Ripe pollen grain of Ciclwi-imn Intybus. The 

 almost splierical cell-wall is beset witii reticulately connected 

 thickening ridges, each of which supports comb-like series of 

 spinose thickenings, projecting still more prominently. 



