652 LABORATORY EXERCISES 



LESSON 40 



Relation of Light to Pathologic Conditions. — While light plays an important part 

 in the development of normal tissue, a lack of it is responsible for man}' abnormal 

 conditions, and there are a number of diseases common to plants under glass which 

 are traceable to insufficient light. Plants, such as cucumber, grown under the poor 

 light common to November and December, have leaves of poor color, slender and 

 elongated petioles, and little mechanic or resistant tissue, and when subjected to the 

 bright sun in the early spring every plant in the house will wilt. Poor light also 

 renders cucumber plants more susceptible to powdery mildew and often causes the 

 tender edges of the leaves to wilt, turn brown and die. The larger number of leaves 

 produced in lettuce plants prevent light from reaching the stem, and stem-rot (Sclero- 

 tinia) or "drop" could undoubtedly be prevented, if the stem were continually 

 exposed to sunlight. The leaf blights of chrysanthemum and tomato, caused by 

 Cylindrosporium, are associated with insufficient light and circulation of air at the 

 base of the stem. Cf. Stone, George E.: The Relation of Light to Greenhouse- 

 Culture. Bull. 144 (July, 1913), Mass. Agric. Exper. Sta. 



Experimental Work. — Grow cucumbers and lettuce plants from seed and expose 

 the potted plants to various light intensities in the greenhouse by shading with 

 several thicknesses of glass, by placing in shaded places in the greenhouse, by growing 

 next to the glass in the best lighted places. Note the effect on the growth and general 

 health of the plants. Grow morning glories in pots during winter and study growth. 



Etiolation and the Health or Vigor of Plants. — In order to study the tonic influence 

 of light upon a plant, we must study its growth in darkness. We find that a plant 

 grown in the dark is modified both in form and structure. The woody and scleren- 

 chymatous elements are much reduced, and the parenchyma of the cortex is in- 

 creased in bulk. The stem becomes very much elongated and remains slender. 

 It is more succulent than a normal stem, and bears extremely small leaves which grow 

 out from it at a more acute angle than those which rise upon a normally illuminated 

 stem. The reaction of its sap is much more acid. The chloroplasts do not become 

 green, the pigment, which they contain, known as etiolin, being a pale yellow. In 

 the leaves, the differentiation of the mesophyll into palisade and spongy parenchyma 

 does not take place. Plants thus affected by darkness are said to be etiolated. 



Experimental Work. — Grow the following plants in light and in total darkness: 

 Arisama triphyllum, Asparagus officinalis, Caladium esculcntum, Castanea dentata, 

 Aesculus hippocastanum, Hyacinthus, Onoclea sensibilis, Osmunda cimtaniomea, 

 Polystichum acrostichoides, Quercus rubra, Sarracenia purpurea, etc. Contrast 

 influence of etiolation by a determination of water content, dried material, ash, 

 starch (by iodine method) duration of etiolated organs and plants, structure of leaves, 

 development of emergences, stomata, lenticels, collenchyma, schlerenchymatous 

 and other histologic structures. Sections can be made by paraffin and celloidin 

 methods, etc. 



LESSON 41 



Withering, or Wilting of Plants. — When the amount of water given off by plants 

 in transpiration is excessive, the leaves and branches lose their turgescence, become 



