CHAPTER II 



DIFFERENTIATION OF THE TISSUES 



General Survey 



A tissue is a group of united cells that are essentially alike, 

 have had a common origin, and are prepared to perform a com- 

 mon function: thus, the outermost layer of cells, or epidermis, 

 of an apple; the groups of bast and wood fibers, the assemblage 

 of cells forming the pith, are tissues. At the growing apex 

 of a stem there is but one tissue (Fig. ii), for the cells are essen- 

 tially all alike. This is known as the primordial meristem. 



On comparing cross and longitudinal sections at successive 

 planes below the apex we find that the primordial meristem 

 soon becomes changed into three distinct parts: the protoderm 

 at the exterior, the procambium strands, and the fundamental 

 or ground meristem (Fig. ii). These three regions or tissues 

 are to undergo further differentiations, and are known as pri- 

 mary meris terns. In still older portions of the stem, lower down, 

 we find that the protoderm has changed into a definite outer 

 skin or epidermis; the procambium has become transformed 

 into three parts, now collectively termed a vascular bundle, 

 an outer part called the phloem, an inner, called the xylem, 

 and a part between these two termed the cambium; and the 

 ground meristem is seen to have become differentiated into 

 four main parts, namely, primary cortex, pericycle, primary 

 medullary rays, and medulla or pith (Figs, ii and 14). These 

 divisions are suggested by distinct landmarks; thus, the inner 

 limit of the primary cortex is a zone of cells, known as the endo- 

 dermis or starch sheath, which is usually characterized by an 

 abundant starch content; the pericycle extends from this to 

 the outer border of the vascular bundles; the primary medul- 



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