26 DIFFERENTIATION OF THE TISSUES 



T , : -.racier oi the different tissues and their origin and 

 \)i\) o. dj\elopment will now be considered. 



.^uiioruial Meristem. — The primordial meristem oc- 

 curs .. xie growing apices, and from it all parts of the plant, 

 dire or ind.rejily, take their origin. The cells composing 

 it .. . .curly isod ametric, are essentially all alike, and are 

 cna .. c.Acd by meir relatively small size, relatively large 

 nil: c dj.ibc c^ . ) ;iasm, absence of vacuoles, absence of insol- 

 uble v).»ds, sucii .. ; starch and oil, very thin cellulose walls, and 

 povvc> Ji repeaxd division. They are of course very tender, 

 hav. i;- liuie sire m and rigidity excepting that due to tur- 

 gid, l/, and ha\'i in and of themselves no protection against 

 losi o. water, Lie; need to be protected when growing in the 

 air, oy me oldj ssues of scales, etc. The primordial meri- 

 stem .s contin ,.. .arried forward by the enlargement of the 

 cells )eneatn i^ .a/e been formed by its own cell division; 



just as a man, vl -; on a stone wall which he is building, 



is carriid upwaiu each successive course of stone. The 



cells a; d tissues . is successively older as they recede from 

 the apex. Some ' \a / in the heredity of the daughter cells 

 of the primordial me' si em causes them soon to become differ- 

 entiated into tii'j ■ primary meristems called the protoderm, 

 projambium, arc! ro "id meristem (Fig. ii). 



ihe Protode m. T.u's is the outermost layer of cells close 

 to', ar d in some u-isiai ces even surrounding the apex, which, 

 after a period of cell d'.ision, is to become the epidermis. In 

 most cases its cell a isions give rise to radial walls only. 

 Radial walls run parallel with a line extending from center to 

 circumference; so that it increases in superficial expanse as the 

 tissues within enlarge; but in some instances it divides tangen- 

 tially (that is, at right angles with the radius) and thus gives 

 rise to a multiple epidermis of several cell layers. Usually the 

 cells of the protoderm differ in form from those of the ground 

 meristem bordering it within, but this is not always the case. 



The Pro cambium.— In dicotyledonous stems the procam- 

 bium occurs as isolated strands disposed in the form of an 



