i84 BOTANY AND PHARMACOGNOSY. 



of the cell is primarily for the purpose of strengthening the walls, 

 but if the walls were uniformly thickened, osmosis, or the trans- 

 ferral of cell-sap from one cell to another, would be hindered. 

 Thus we find that the contiguous walls of the cells are thickened 

 at definite places opposite each other, leaving pores or canals 

 which permit rapid osmosis. The pores thus formed are known 

 as simple pores, and when seen in surface view are somewhat 

 elliptical or circular in outline, and may be mistaken for some of 

 the cell-contents. These thickenings assume a number of forms, 

 which are quite characteristic for the plants in which they are 

 found. They may have the form of transverse or oblique rings, 

 longitudinal spirals, or be ladder-like or reticulate in appearance 

 (Fig. 102). In other instances the thickening of the wall is quite 

 complex, as in the wood of the pines and other Coniferse (Fig. 

 103). The thickening, or sculpturing, as it is sometimes called, 

 may not only occur on the inner surface of the wall, when it is 

 spoken of as centripetal, but may also take place on the outer 

 surface, when it is known as centrifugal; as examples of the 

 latter, may be mentioned the spores of lycopodium (Fig. 278b) 

 and the pollen grains of the Compositse (Fig. 280). 



FORMS OF CELLS. 



Upon examining sections of various portions of the plant, it 

 is observed that not only do the cell-contents and cell wall vary 

 in composition, but that the cells are of different forms, depend- 

 ing more or less upon their functions. Groups of cells which 

 are similar in form and function constitute the various tissues of 

 the plant; and they may be classified, for convenience of study, 

 as follows: (i) parenchyma cells, (2) mechanical cells, (3) 

 conducting cells and (4) protective cells, 



Parenchyma. — Under the head of parenchyma are included 

 those cells which are nearly isodiametric and thin-walled, the walls 

 consisting of cellulose lamellae (Fig. loi, A). They may contain 

 both protoplasmic and non-protoplasmic cell-contents. Accord- 

 ing to the function and nature of contents, three kinds of paren- 

 chyma cells are recognized : (a) Chlorophyll-parenchyma or 

 assimilation parenchyma contains numerous chloroplastids and 



