60 Introductory Biology 

 Kinds of Plant Tissues (Cont'd) 



SHAPE, SIZE, AND ARRANGE- 

 MENT OF CELLS 



LOCATION AND 

 FUNCTIONS 



8. PHLOEM 



Complex tissues composed of several 

 kinds of cells; phloem always 

 contains sieve tubes and paren- 

 chyma, and other three kinds of 

 cells described may, or may not, 

 be present, depending on specific 

 tissue. Phloem may be com- 

 posed of: 



(a) Sieve tubes, which are elongated 

 rows of thin-walled, cylindroid 

 living cells whose end walls 

 (sieve plates) contain sievelike 

 pores; protoplasm continuous 

 from cell to cell through sieve 

 pores; in mature sieve tube cells 

 living protoplasm lacks nuclei. 



(b) Companion cells, which are ad- 

 jacent to sieve tubes, are some- 

 what shorter and smaller than 

 latter, and possess prominent 

 nuclei. 



(c) Phloem parenchytna much like 

 ordinary parenchyma and always 

 present in phloem. 



(d) Phloem ray cells which are 

 parenchymatous to conduct ma- 

 terials radially. 



(e) Phloem fibers which are elon- 

 gated cells whose structure gives 

 strength. 



Phloem conducts foods 

 manufactured in leaves 

 downward through stems 

 and roots; companion 

 cells common in flowering 

 plants and because of 

 pores between them and 

 sieve tubes, former may 

 assist in conducting and 

 storing foods; phloem 

 parenchyma stores foods; 

 phloem ray cells store 

 foods and conduct mate- 

 rials radially in stems, 

 etc. ; phloem fibers give 

 support and strength. 



III. ORGANS 



An organ is an association of different tissues which act together to 

 perform some specific function. For instance, the human arm is an 

 organ of motion consisting of such tissues as bone, cartilage, muscle, 

 blood and lymph, connective, vascular (blood vessels), nervous, epithe- 

 lial, and adipose. Many organs are usually required for performing a 

 particular function, each organ contributing some part, large or small, 

 to the functioning of the whole. All organs tend to function together 

 in a more or less harmonious manner if the living organism is normal 

 and healthy. If ill or defective, there is a maladjustment of the interac- 

 tion and interdependence of the various organs of that individual. 



The leaf of a plant is an organ, composed of such tissues as epidermal, 

 chlorenchyma, collenchyma, xylem, phloem, etc. 



