22 



MORPHOLOGV OF THE CELL. 



calary and terminal growth of the cell-wall. Terminal growth takes place when the 

 surface-growth attains a maximum at any one part of the circumference (by inter- 

 position of new particles of cell-wall), while the intensity of this process decreases on all 

 sides of this point, and at a definite distance attains a minimum, so that this portion of 

 the cell- wall projects as a point, or appears as the rounded end of a prominence, or of a 

 cylindrical sac {e. g. hairs, filamentous Algae). If several points of terminal growth occur 

 in a cell which was originally round, it may become star-shaped ; if new points of growth 

 are formed beneath the continuously growing end of sac, the sac-like cell branches (as in 

 many filamentous Algae, hyphae of Fungi, Vaucheria, Bryopsis). Hofmeister^ distinguishes 

 as a peculiar form of terminal growth the case in w^hich, instead of a point, a line is 

 rapidly raised on the cell-wall ; this may occur as the terminal line or intersecting edge 

 of two curved surfaces. Intercalary growth of the cell- wall 

 occurs in a typical form in the case in which the deposition of 

 new substance within a belt lying in the surface of a cell takes 

 place in such a manner that this belt extends, and a fresh 

 interposed piece of the cell-wall from time to time makes its 

 appearance. To the last-named case may be referred the 

 common phenomenon of the occurrence of growth in the 

 whole of the side-wall of a cubical, tabular, or cylindrical cell, 

 as, for example, in the cells of Spirogyra, and the parenchyma- 

 cells of growing roots and stems of Phanerogams (cf. Fig. i). 

 Oedogonium presents a peculiar case of intercalary surface- 

 growth (Fig. 17). Inside below the septum is formed a project- 

 ing annular cylindrical deposit of cellulose {A, qu) ; there the 

 cell-wall splits, as if separated by a circular cut, into two 

 pieces ; and these now, retreating from one another, remain 

 united by a zone of cell-wall {B, nv') formed by extension of 

 the cylinder ^u. After the interposition of this new cylindrical 

 zone, cell-division follows ; and, sipce this is repeated many 

 times, the appearance is presented which is figured at A, c 

 (the so-called formation of a cap'). 



(b) The Gro^vth in Thickness of a Cell-^joall is usually 

 strictly localised, so that the thicker parts appear mostly as 

 very abrupt projections on the thinner parts of the cell-wall, 

 either on the outside or the inside. The collective impres- 

 sion made by the sculpture is especially dependent on 

 whether the extension of surface is less on the thicker or 

 on the thinner parts. If the thickening is especially strong 

 on certain points, the structure takes the form outwardly 

 (Fig. iq) or inwardly (Fig. 18, C, D) of projecting warts, 

 pegs, or spines ; if the thickening occurs most strongly in 

 linear or strap-shaped spots of the cell-wall, projecting cylinders, ridges, bands, or combs 

 are formed on the inner or outer side. These ridge-like projections may form reticu- 

 lated figures on the inner or outer side (Fig. 18, 5, Fig. 20, /), or rings, or spiral bands, 

 a development especially frequent in those thickenings of certain tissue-cells which project 

 from within. If the internally projecting rings or spiral bands are thick and firm, and the 

 intermediate portions of cell-wall thin and easily destructible, these thickenings may become 

 free even within the plant, and remain lying as isolated threads of cellulose in channels of 



-Intercalary surfat 

 of Oedogoniiini. 



* Handbuch der physiol. Botanik, 1. p. 162. 



2 For further details of these somewhat complicated processes see Pringsheim, Jahrbuch fur 

 wissen. Bot. 1; Hofmeister, Handbuch der phys. Bot. I. p. 154, and Niigeli und Schwendener, 

 Mikroskop, II. p. 549. 



