TISSUE. 97 



the parenehyiha-cells immediately under the wound become 

 a mass of phellogen, and a protecting mass of cork, called 

 ■wound-cork, is developed. The epidermis may be replaced 

 entirely by a continuous corky layer, called the periderm 

 (Gr. peri, around). Restricted corky growths below sto- 

 mates may push out the epidermis, as in Elder ; and thus 

 roundish, or elongated masses of cork, called lenticels, are 

 formed. 



123. At the growing ends of stems and leaves (and 

 roots) is found a tissue composed of cells, with delicate 

 walls, filled with protoplasm, capable of division. This is 

 called primary meristem (Gr. meros, part ; 

 temnein, to cut ofT^, and from it the various 

 tissues are developed. As growth proceeds, 

 portions of the primary meristem. become 

 transformed into permanent tissue, which is 

 incapable of division. The terminal portion 

 of an organ consisting of primary meristem, 

 and having permanent apical growth, is called ^^'' ^^' 

 the punctum vegetationis, or growing point. As it 

 often projects in a conical elongation it is sometimes called 

 the vegetative cone. The punctum veffetationis may con- 

 sist of a single cell, called the apical cell (Figs. 217, 218, 

 op), or it may be composed of a multitude of cells (Fig. 

 219). Many of the Algae grow from an apical cell. This 

 . elongates and divides, the upper portion continuing as the 

 apical cell, which, in turn, elongates, and divides as before ; 

 and so on. The other portion, appearing like a piece or 

 disc cut ofi" from the apical cell, is called a segment. The 

 segments may remain undivided (Fig. 217). They may 



Figs. 217, 218. Diagrams showing growth from an apical cell ; segments (^ /^ 

 ///, IV) divided (3iS) and undivided [ziT). 

 7 



