CHAPTER VII 



Wall Structure in Thick Cell Walls. Flowering Plants 



Layering in xylem cells 



TURNING, THEN, to the morc familiar types of plants it is found that, 

 in spite of the very different form of the cells involved, and in spite 

 of their very different method of development, the fine structure of the 

 walls is in broad outline remarkably similar. We may perhaps turn our 

 attention first to those features which are visible microscopically, and 

 examine in the first place the tracheids which are supposed to be the 

 primitive cell type, from forms like which the more specialized cells of 

 the xylem have developed. Tracheids can be recognized in macerated 

 material as elongated cells with tapering, but seldom pointed, ends with 

 moderately thick cell walls and prominent pits. 

 In transverse section it is sometimes difficult to 

 distinguish these from fibres if the section has 

 not included a pit, so that it will be as well to 

 take up this particular aspect of the study 

 with the wood of conifers since, apart 

 from parenchyma cells and the ray tissue 

 with which there can be no confusion, 

 the wood is composed exclusively of tra- 

 cheids. 



If a thin transverse section of the wood of 

 a conifer is examined under the microscope 

 then it can be seen, particularly if the section 

 has been stained in safranin or congo red, that 

 the wall is layered (Fig. 40; Plate V, Fig. 3), 

 Commonly, three such layers are visible in each 

 wall (and therefore six in the double wall 

 between each pair of cells) whose extent depends 

 on the region of the wood from which the 

 section is taken. In the thicker walled late wood produced towards the 

 end of the growing season, the central layer is thick and the outer and 

 inner layers comparatively thin. In the thinner walled early wood the 

 central layer too is thin and, in fact, may be thinner than either of 

 8 113 



Fig. 40. Diagrammatic re- 

 presentation of a trans- 

 verse section of a conifer 

 tracheid in late wood, to- 

 gether with parts of the 

 contiguous cells. The cells 

 are held together by a 

 middle lamella; the secon- 

 dary wall is divided into 

 three lamellae — the inner 

 and outer ones narrow and 

 the central one thicker. 

 The primary wall is too 

 thin to show on this scale. 



