LABORATORY STUDIES 41 



(<:•) Study tlie foregoing types of tracheary tissue in cross- 

 section in comparison with the longitudinal sections. 



((/) The larger pores in the wood of oak, hickory, etc., as 

 well as in the grape, are pitted vessels. 



(e) Excellent scalariform vessels are to be found in the 

 leaf-stalks or better still, in the underground stems of the 

 brake {Ptcridium aquilinum). 



(/) The tracheids of pine, spruce, etc., resembling wood fibers 

 in shape, but with bordered pits, should be studied by making 

 tangential and radial longitudinal sections as well as cross- 

 sections of the wood. The bordered pits occur only on the 

 radial surfaces of the tracheids. 



{g) Spirally marked tracheids, similar in shape to the fore- 

 going, may be found in the wood of the hackberry (Celtis), 

 and ash. 



{h) By treating various kinds of wood with Schulze's reagent 

 (nitric acid and potassium chlorate, warmed) the various cells 

 will be separated and the tracheary elements of different kinds 

 can be studied separately. 



{%) Sieve tissue is easily found by making longitudinal sec- 

 tions of the stems of squashes or pumpkins (Cucurbita) or 

 other vines such as grape, clematis, hop, etc. They will be 

 found in the part of the vascular bundle Ij'ing toward the 

 outside of the stem and in the case of Cucurbita also on the 

 inner side. By staining with eosin or carmine, the protoplasm 

 and protein contents will be stained. If alcoholic material be 

 used, the contents will be found shrunken away from the sieve 

 plates. If portions of living stems are killed before sectioning 

 by dipping into very hot water, the protein and protoplasmic 

 contents will be coagulated without much contraction. 



ij) Make numerous very thin cross-sections of the same 

 stems and examine until sieve plates are found and studied in 

 surface view. 



{k) Examine a drop of latex from milkweed, spurge or poppy, 

 under high magnification. The suspended granules will be 

 visible as fine dark brown bodies by transmitted light. Test 

 with iodine to determine whether starch grains are present. 



(/) Collect a quantity of latex of spurge (Euphorbia) and 

 let it evaporate in a watch glass. The residue is a sticky, 

 rubbery mass, which on being burned, has the characteristic 

 odor of burning rubber. 



