26 DIFFERENTIATION OF THE TISSUES 



The character of the different tissues and their origin and 

 progress of development will now be considered. 



The Primordial Meristem. The primordial meristem oc- 

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

 directly or indirectly, take their origin. The cells composing 

 it are nearly isodiametric, are essentially all alike, and are 

 characterized by their relatively small size, relatively large 

 nuclei, dense cytoplasm, absence of vacuoles, absence of insol- 

 uble foods, such as starch and oil, very thin cellulose walls, and 

 power of repeated division. They are of course very tender, 

 having little strength and rigidity excepting that due to tur- 

 gidity; and having in and of themselves no protection against 

 loss of water, they need to be protected when growing in the 

 air, by the older tissues of scales, etc. The primordial meri- 

 stem is continually carried forward by the enlargement of the 

 cells beneath it that have been formed by its own cell division; 

 just as a man, standing on a stone wall which he is building, 

 is carried upward with each successive course of stone. The 

 cells and tissues are thus successively older as they recede from 

 the apex. Some quality in the heredity of the daughter cells 

 of the primordial meristem causes them soon to become differ- 

 entiated into the three primary meristems called the protoderm, 

 procambium, and ground meristem (Fig. u). 



The Protoderm. This is the outermost layer of cells close 

 to, and in some instances even surrounding the apex, which, 

 after a period of cell division, is to become the epidermis. In 

 most cases its cell divisions 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 Procambium. In dicotyledonous stems the procam- 

 bium occurs as isolated strands disposed in the form of an 



