104 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[24. 



FIG. 57. Ripe pollen-grain 

 of Cichorium Intybus ; the al- 

 most spherical surface of the 

 cell -wall is furnished with 

 ridge-like projections pro- 

 longed into spines, and form- 

 ing a network. (After Sachs.) 



until after its growth in surface has ceased, the cell having then 

 attained its definite size ; but cases of simultaneous growth in 

 surface and in thickness have been ob- 

 served. 



The growth in thickness of the cell- 

 wall is also rarely uniform ; the cell-wall 

 commonly becomes more thickened at 

 some points than at others, and thus 

 acquires inequalities of surface. In the 

 case of isolated cells or of free cell-walls, 

 the prominences existing in this way on 

 the external surface appear as warts, 

 tubercles, spines, etc. (Fig. 57). Cells 

 that are united to form tissues have their 

 inequalities on the internal surface of the 

 cell- wall, the prominences sometimes having definite form, and 

 projecting into the interior of the cell ; such are the annular 

 (Fig. 58 r} and spiral thickening (Fig. 58 s) of the walls of certain 

 vessels ; in the so-called reticulated cell- walls, the thickening is 

 in bands which are united into a network, so that circular or 

 oval thin spaces are left. In other cases, 

 solitary and relatively small thin spaces 

 are left in the wall in the course of the 

 growth in thickness, which appear, when 

 seen on the external surface, as bright 

 spots, commonly called pits, and are seen 

 in section to be canals of greater or less 

 length, according to the relative thick- 

 ness of the walls (Figs. 59, 60). Very 

 frequently the pit, when seen from the 

 surface, presents the appearance of two 

 concentric circles, or ellipses ; for this 

 reason, that the opening of the canal into 

 the interior of the cell is narrow, whereas 

 the external opening is broad (Fig. 62 A). 

 Such bordered pits occur in the wood-cells 

 of Conifers (Fig. 63), in the walls of 

 many vessels (Fig. 62), and elsewhere. 

 The scalariform thickening of the walls of 

 many vessels arises from the regular and close arrangement of 

 bordered pits which are much elongated transversely. 



FIG. 58. r Annular, s spiral 

 thickening of the walls of ves- 

 sels ; r seen from outside, s in 

 longitudinal section highly 

 magnified (diagrammatic). 



