28 DIFFERENTIATION OF THE TISSUES 



derived; and cells of the medullary rays in like manner produce 

 cambium cells {interfascicular cambium) which, joining with 

 the cambium of the vascular- bundles {fascicular cambium) 

 complete the cambium ring. The interfascicular cambium and 

 the cork cambium are classed as secondary meristem. The 

 details of their origin and their relation to the other tissues will 

 appear under the discussion of secondary increase in thickness. 



The Primary Pennanent Tissues. — The tissues formed by 

 the transition of the primary meristems into the permanent 

 state where cell division ceases are known as the primary per- 

 manent tissues in contradistinction to those that are the product 

 of cambium cells, which are called secondary permanent tissues. 



Beginning at the outside, the primary permanent tissues in 

 dicotyledons are grouped on anatomical and topographical 

 grounds into the following main divisions: Epidermis, primary 

 cortex, pericycle, phloem part of the vascular bundle, xylem 

 part of the vascular bundle, medullary ray, pith (Fig. 14). In 

 Monocotyledons the order is the same until the pericycle is 

 passed, and then follow the vascular bundles, with phloem 

 facing outward and xylem inward, scattered through the ground 

 tissue, where, in consequence, medullary rays and pith are not 

 distinguishable (Fig. 28). The tissue groups in dicotyledons 

 will now be taken up in the order named. 



Epidermis. ^^Ks the protoderm cells are becoming trans- 

 formed into the epidermis they enlarge more or less, varying 

 a great deal in this respect in different plants and habitats, and 

 in different parts of the same plant; and they assume a diver- 

 sity of forms, from isodiametric to much elongated, and from 

 regular polyhedra to forms of sinous contour (Fig. 12); and 

 they may even grow out from the surface in the form of scales 

 and hairs. 



While the changes in size and form are going on, the outer 

 wall is, as a rule, thickening and becoming chemically altered, 

 particularly in parts exposed to the air. The most important 

 chemical change is due to the addition and infiltration of a 

 waxy substance called cutin which makes the outer wall water- 



