THE CELL. 33 



localized surface growth which causes the apical growth of a cell, 

 and will now treat principally of growth in thickness. 

 As to development two forms can be recognized : 



(a) At points of active cell-formation (ineristem of apical areas) 

 division proceeds so rapidly and the growth in thickness of cell- 

 walls is so slight and so soon completed that all membranes have 

 nearly the same immeasurable thinness. Only after cell-formation 

 ceases or becomes less frequent, that is, at some distance from the 

 apical area, does the characteristic thickening of cell-walls begin. 



(b) Less commonly growth in thickness begins and continues 

 immediately after the formation of the cell. In these cases the rela- 

 tive age of the cell-wall can be estimated by its thickness (examples : 

 algal threads and cork-formation). 



As has been stated, growth in thickness may be uniform and 

 may show all gradations from minimum to maximum, that is, up to 

 total occlusion of the cell-lumen (Fig. 13, <z, 5, <?). 



Of the very frequent unequal cell-wall thickening I will men- 

 tion first the u collenchymatous." Cells showing this thickening 

 belong to the mechanical tissues and are characterized by thick 

 angles (Fig. 14). The selection of the name " collenchyma " 

 (^o^Atf, lime) probably depends upon the fact 

 that certain cells with walls of unequal thickness 

 and at the same time gelatinous were so named. 

 Collenchyma is, however, not capable of swelling 

 to any considerable extent (AMBRONN). 



The other forms of localized growth in thick- 

 ness of cell-walls may be grouped as follows : FIG. 14. 



I. Spine or wart-like thickenings (usually projecting outward). 



II. Linear or fibrous thickenings (usually projecting inward). 



III. "Porous" thickenings; that is, thickenings with the ex- 

 ception of certain areas called " pores." 



