284 



PLANT STEUCTUEES 



below them are the three embryonic regions, dermatogen, 

 periblem, and plerome ; and farther below these three 

 regions pass into permanent tissue, organizing the epider- 

 mis, cortex, and stele . The three embryonic regions are 

 usually not so distinct in the Conifer stem as in the Dico- 

 tyl stem, but both stems have epidermis, cortex, and stele. 

 Epidermis. — The epid ermis is a protective laye r, whose 

 cells do not become so much modified but that they may 

 be regarded as parenchyma. It gives rise also to super- 

 ficial parts, as hairs, etc. In the case of trees, the epidermis 

 does not usually keep up with the increasing diameter, and 

 disappears. This puts the work of protection upon the 

 cortex, which organizes a superficial tissue called cork, a 

 prominent part of the structure known as bark. 



Cortex. — The cortex is characterized by containing 

 much active parenchyma, or primitive tissue, being the 

 chief seat of the life activities of the stem. Its superficial 

 cells, at least, contain chlorophyll and do chlorophyll work, 

 while its deeper cells are usually temporary storage places 

 for food. The cortex is also char- 

 acterized by the development of 

 stereome, or rigid tissues for me- 

 chanical support. The stereome 

 may brace the epidermis, forming 

 the hypodermis ; or it may form 

 bands and strands within the cor- 

 tex ; in fact, its amount and ar- 

 rangement differ widely in differ- 

 ent plants. 



The two principal stereome tis- 

 sues are collenchyma and sclereii- 

 cliyma, meaning "sheath-tissue" 

 and " hard-tissue " respectively. 

 In collenchyma the cells are thick- 

 ened at the angles and have very elastic walls (Fig. 267), 

 making the tissue well adapted for parts which are growing 



Fig. 267. Some collenchyma 

 cells from the stem of a com- 

 mon dock (Rumex), showing 

 the cells thickened at the 

 angles. — Chamberlain. 



