LABORATORY AND TEACHING METHODS 653 



flaccid and droop, in other words they wilt, or wither. This withering may be due 

 to the lack of water in sufficient quantities, in the soil, or it may be due to the pres- 

 ence of salts of high osmotic equivalent in the soil, which render the absorption of 

 water difficult, or impossible. Plasmolysis may induce wilting. 



Experimental Study. — Take two potted plants and wrap the pot in rubber dam, 

 or oiled paper, so as to cover the pot and soil to prevent evaporation from their 

 surfaces. Weigh both potted plants carefully. Water one each day with a meas- 

 ured quantity of water and let the other remain imwatered until the plant begins to 

 wilt, then weigh it carefully to determine the amount of available water transpired. 

 Then knock out the plant and weigh the soil after drying in an oven to determine 

 the amount of hygroscopic water present. 



We now make the following very instructive experiment with Helianthus litbero- 

 sits. We bend down a long shoot without separating it from the plant, and without 

 cracking it, so that a portion 20 cm. from the summit dips into water contained in a 

 vessel placed below it, the summit of the stem and the leaves not being wetted. 

 We cut through the stem with a sharp knife under water, so that the cut surface 

 remains under water. Our shoot keeps fresh for days, while other Helianthus 

 shoots cut off in the air, and then at once placed in water, rapidly wither. We may 

 make them turgescent again by placing a withered shoot in the shorter limb of a 

 U-shaped glass tube containing water fixed in place in the tube by a rubber 

 cork fitted air-tight about the stem. Mercury is now poured into the longer limb 

 of the tnbe and its pressure is sufficient to revive the withered shoot. Consult 

 Shive, John W. and Livingston, B. E.: The Relation of Wilting Plants. The 

 Plant World, No. 4, April, 1914: 81-129. 



Plasmolysis and Wilting. — Prepare 250 c.c. of 0.5 gram-molecular (M) solutions 

 of potassium nitrate and of sodium chlorid as stock solutions. From these solutions 

 make dilutions in small vials, capacity about 25 c.c. to contain the following strengths 

 of each of the above solutions, namely o.io, 0.20, 0.30, and 0.40 molecular (M); also 

 one vial with distilled water as a control. In each of the dilutions place a seedling 

 of some plant (root as nearly entire as possible) with delicate stem or leaf stalks, 

 such as lettuce, radish or mustard. Water plants can also be used, such as Elodea 

 gigantea, Vallisneria spiralis, Trianea bogotensis and the staminal hairs of Trades- 

 cantea and the filaments of Spirogyra nitida. Observe the dilutions in which wilting 

 occurs and note the time required in the solutions in which it occurs. Compare 

 the equivalent strengths of the two salts (The Country Gentleman, Dec. 6, 1913: 

 1781). 



LESSON 42 



Methods of Sectioning. — By the time that this lesson is reached some of the plants 

 which have been wounded or have been inoculated with the various bacterial and 

 fungous organisms, or have been treated in various ways experimentally, will begin 

 to show growth reactions. Such material can be studied by the making and mount- 

 ing of sections. The sections can be made in one of three ways: (i) By free-hand 

 sectioning, the razor ground flat on one side being held in the hand; (2) by the slid- 



