CONDUCTIVE TISSUES 29 



berry which bend over and take root at the tip and send up new 

 shoots. Stem tubers, which are short and very much thickened 

 rhizomes; corms, which are short and thick but vertical underground 

 stems; and bulbs, which are large buds with very thick bud scales; 

 are all of some importance in vegetative reproduction but are more 

 important as storage organs. 



18. Conductive Tissues.— The conducting elements in the higher 

 plants are largely of four kinds: namely, tracheje, tracheids, sieve 

 tubes and parenchyma. Sap passes upward through the plant 

 largely through tracheae and tracheids. It passes downward mostly 

 through sieve tubes, while lateral passage of sap is primarily through 

 parenchyma tissue, such as that found in medullary rays. 



Tracheae are syncytes; that is, each one is made up of several 

 cells placed end-to-end, but from which the separating walls have 

 disappeared. They usually have pointed ends and are in most cases 

 only a few centimeters in length, though sometimes they are as 

 much as a meter or more long, especially in some climbing plants. 

 Tracheids differ from tracheae in being single cells. They have 

 pointed ends like the tracheae but are usually only a millimeter or 

 less in length, though in some plants they are several centimeters 

 long. Both tracheae and tracheids function as dead empty cells. 

 They both occur in the x^'lem of the fibro-vascular bundles. 



Sieve tubes, which occur in the phloem of the fibro-vascular bun- 

 dles, resemble tracheae in being syncytes; but they differ from 

 tracheae in functioning as living cells with protoplasmic contents, 

 and in having perforated cross walls called sieve plates. 



Water seems to be the chief external factor influencing the devel- 

 opment of conductive tissues. The lower plants, most of which live 

 where there is an abundance of water, if not actually in water, have 

 no special conductive tissue. Also, conducting tissue is greatly 

 reduced or practically absent in submerged seed plants. 



In the case of amphibious plants different individuals of the same 

 species, or even different parts of the same individual, may differ 

 greatly in the relative amoimts of conducti^'e tissue, it being much 

 less developed in those growing in water than in those projecting 

 into the air. Furthermore, it is found that in land plants the maxi- 

 mum amount of conductive tissue occurs in those growing in \ery 

 dry situations and that the moister the environment the less is this 

 tissue developed. This effect is also seen in the annual rings in the 

 wood of trees, much thicker rings being produced in wet years than 

 in dry years. 



