ILLUSTRATIVE STUDIES 121 



lary ray and tracheids join. Determine the number of med- 

 ullary rays in one sq. mm. Discover whether every tracheid 

 is touched once or more than once throughout its length by 

 a ray. 



4. Criticize Fig. 52, which is constructed from the three sec- 

 tions above studied. 



5. Study cross and longitudinal radial and tangential sec- 

 tions of oak, and yellow poplar, double-stained and permanently 

 mounted. Note by comparison of all sections what provision 

 has been made for the interchange of materials between the tra- 

 cheal tubes, tracheids, medullary rays, wood parenchyma and 

 wood fibers. 



In the tangential sections determine how many rays occur in 

 one sq. mm. and how many rays touch a tracheal tube for every 

 mm. of its ascent. The purpose of this is to find out how com- 

 plete is the provision for taking up and distributing the water 

 and solutes carried vertically by the tracheal tubes. 



6. Compare for frequency and water-carrying capacity the 

 tracheal elements in cross sections of stems of oak, hop and 

 water lily. Make diagrams to illustrate what is found. 



7. Study the tracheal tissues in leaves. Clear a leaf by boil- 

 ing it in alcohol, placing it in 5 per cent, hydrochloric acid for 

 about ten hours or over night, and keeping it for a while in a 

 saturated chloral hydrate solution. Mount the leaf in chloral 

 hydrate. The leaf should now be perfectly clear. Study it 

 with low and high powers. Make drawings of the endings of 

 the tracheal tissues of the veins. Measure the distance apart 

 of the ultimate branches of the veins. Determine by slow focus- 

 ing from one surface to the other what position the tracheal 

 tissues occupy in the leaf. We are in this way finding evidence 

 of the efficiency of the tracheal elements in distributing water 

 quickly to all cells of the leaf. 



8. Select a plant having leaves with prominent veins. Cut 

 some of the veins across clear through. Does the leaf above 

 the cut wilt? Cut across more and more veins. How do you 

 account for the results? 



