Cellular Organization of Plants and Animals 55 



White matter 



dorsal horn 



MnS 



lateral horn 



Musda 



] Ventral root 

 5pinal nerve 



\/ 



\jQr\bra\ horn 



qray matter 



Fig. 17. — Spinal cord in cross section, showing the pathways through the cord 

 and the origin of the spinal nerves. Arrows indicate the pathways over which 

 impulses might travel from the skin through the cord and back to a muscle. 



II. PLANT TISSUES 



A plant tissue is a group of cells commonly of similar structure and 

 performing essentially the same function. An organ is composed of vari- 

 ous tissues which together perform interrelated functions. For example, 

 a leaf is an organ composed of various types of tissues. Naturally, the 

 simpler, lower types of plants do not have tissues, or, if they do, the tis- 

 sues are quite simple. The cells of plants show great variations in struc- 

 ture and size, both of which influence the functions of these cells in the 

 physiology of the plant. Plant tissues may be simple or complex (Fig. 

 18). The phloem and xylem tissues are considered to be complex be- 

 cause several kinds of cells occur in their construction, while other tissues 

 described below are considered to be simple because they are composed 

 of a single kind of cell. Certain tissues, such as the rapidly growing 

 meristematic tissues, give rise to other tissues and might be considered 

 temporary, while the other tissues remain much the same after being 

 formed and might be considered permanent tissues. When tissues are 

 studied casually and hurriedly, they may appear to be alike, but a scien- 

 tific, microscopic study shows that because of differences they may be 

 classified into rather different types (Fig. 18). 



Types of Plant Tissues (Figs. 18 and 19) 



1. MERISTEMATIC (mer is te -mat' ik) (Gr. meristes, divide) 



2. EPIDERMAL (ep i -der' mal) (Gr. epi, upon; derma, skin) 



3. PARENCHYMA (par -eng' ki ma) (Gr. para, beside; engchyma, infusion) 



4. COLLENCHYMA (kol -eng' ki ma) (Gr. kolla, glue; engchyma, infusion) 



5. SCLERENCHYMA (skier -eng' ki ma) (Gr. skleros, hard; engchyma, infusion) 



A. SCLERENCHYMA FIBERS 



B. STONE CELLS (SCLEREIDS) 



6. CORK (kork) (Span, alcorque, cork) 



