← Handbook of nature-study for teachers and parents, Volume 2
Chapter 78
Lesson CXLII
Prickly Lettuce
Leading thought—The sunshine sets the machinery in the leaf-factories going, and incidentally pumps up water from the soil, which pours out into the air from the leaves; but if the soil is dry the pump works just the same, and the plant thus robbed of its water soon withers and dies. The young plants of wild lettuce prevent the sun from pumping them dry during drought, by turning the edges of their leaves toward the sun, and thus not exposing the leaf surface to its rays. The leaves thus lifted stand in one plane. They are usually directed north and south. The lettuce also has spines to protect it from grazing animals.
Method—The lettuce should be studied in the field, and is a good subject for a lesson in late summer or September. This lesson should supplement the one on transpiration. The young plants show this arrangement of the leaves best. The flowers may be studied by the outline given in Lesson CXXXV.
Observations—1. Where does the prickly lettuce grow? What sort of a stem has it? How are the leaves arranged on the stem?
2. If the leaves stood straight out from the stem, what would be the shape of the plant? How do the leaves stand? Is their upper surface exposed to the rays of the sun? Which portion of the leaf is turned toward the sun?
3. If the leaves turn sideways and stand in one plane, do they stand north and south or east and west. How does the edgewise position of the leaf protect the plant during drought? Why does any plant wither during drought? If the leaves of the lettuce should extend east and west instead of north and south, would they get more sun? (See lesson on the Sun.)
4. What is the shape of the lettuce leaf? How does it clasp the stalk? How is the base shaped so that the leaf can turn without tearing its edges? Sketch a leaf thus turned fully, showing how it is done. Does the leaf turn toward the stem or away from it?
5. How are the leaves protected against grazing cattle? How does the cow use her tongue to help bring herbage to her mouth? How are the prickly spines placed on the lettuce leaf, to make the cow’s tongue uncomfortable? Sketch a leaf showing its shape, its venation and its spines.