TRACHEARY TISSUE 35 



those cells that happen to be so placed that a thickened angle 

 appears in the section will show this feature. On the other 

 hand, if the section passes between the corners of the cell the 

 walls will appear thin. 



(</) Collenchyma may be found also in the young green shoots 

 of elder (Sambucus) and some other shrubs, in the stems of 

 lamb's quarters (Chenopodium), pigweed (Amaranthus), 

 petioles of beets and very many other plants. 



(h) Make thin longitudinal sections of the wood and bark of 

 the basswood (Tilia) or maple (Acer) and macerate, to 

 separate the cells, in Schulze's reagent (i.e. heat in a test tube in 

 nitric acid to which has been added a little potassium chlorate). 

 Mount a bit of the macerated wood section on a slide and tap 

 the cover glass, or tease the section apart with needles. 

 Study the wood fibers. Do the same for the bast fibers in the 

 bark. 



(i) Now make thin longitudinal and cross-sections of the 

 same kind of twig without macerating and study the fibers in 

 place to note the relation of the overlapping cells. In the cross- 

 section, note the appearance of the fibers and their position in 

 the twig. 



45. Besides the foregoing, there is a group of tissues 

 which have as their chief function the conduction of 

 water and food, the so-called conductive tissues. These 

 are of three kinds: tracheary tissue, whose primary func- 

 tion is the transportation of water, and sieve and lat- 

 iciferous tissues, which are chiefly concerned with the 

 conduction of food substances manufactured by the 

 leaves. 



46. Tracheary tissue is of many kinds. The term is here 

 used to include those elongated cells, whose chief function 

 is the transport or storage of water. The lumen is usually 

 rather large with the wall thickened in a more or less regu- 

 lar manner to give strength. At the same time, a consider- 

 able portion of the wall remains thin, permitting the en- 

 trance or exit of water. The cells are not living, i.e. their 

 protoplasm dies as soon as they have attained their final 



