74 



PART II. ANATOMY AND HISTOLOGY. 



[23 



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. 43). 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. 44 r) and 

 spiral thickening (Fig. 44 ) 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, isolated 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 

 thickness of the walls (Figs. 45, 46). Very 

 frequently the pit, when_seen from the 

 surface, presents the appearance of two 

 concentric circles, or ellipsesj_j.or this 

 reason, that the opening of the .canal into 

 the interior of the cell is .narrow, whereas 

 the external opening is broad (Fig. 48 A}. 

 Such bordered pits occur in the wood-cells 

 of Conifers (Fig. 49), in the walls of many 

 vessels (Fig. 48), and elsewhere. The 

 scalar if orm thickening of the walls of 

 many vessels arises from the regular and 

 close arrangement of bordered pits which 

 are much elongated transversely. 



The Structure of the Cell-icall. When 

 the cell- wall is at all thickened it presents indications of structure. 

 It presents, in the first place, a layered appearance when ex- 

 amined in longitudinal or transverse action (Fig. 46). This layer- 

 ing or stratification of the cell-wall is readily intelligible when it 

 is remembered that the thickening of the wail is due to the depo- 

 sition of successive layers from within. 



It presents, secondly, a delicate striation, when examined in 

 surface-view, the lines running at a larger or smaller angle to the 

 long axis of the cell, sometimes even transversely. The planes of 

 striation are commonly different in the different layers constitu- 



Fia. U.r Annular, spiral 

 thickening of the walls of ves- 

 sels ; r seen from outside, s in 

 longitudinal section highly 

 magnified (diagrammatic). 



