The more we know of plants, the more we admire their ways of attaining success in a world where it is only attained by a species after a long struggle. While plants may not be conscious of their own efforts for living on successfully, they have developed them just the same, and they merit our admiration perhaps even more, than as if their strategy was the result of conscious thought. The prickly lettuce has a story to tell us about success attained by the prevention of exhaustion from thirst. In fact, the success of this weed depends much upon its being able to live in dry situations and withstand the long droughts of late summer. The pale green stems grow up slim and tall, bearing leaves arranged alternately and from all sides, since between two, one of which is exactly above the other, two other leaves are borne. Thus, if the leaves stood out naturally, the shape of the whole plant would be a somewhat blunt pyramid. But during the hot, dry weather, the leaves do not stand out straight from the stem; instead, they twist about so that they are practically all in one plane, and usually point north and south, although this is not invariably the case. The way this twisting is accomplished is what interests us in this plant. The long spatulate leaf has a thick, fleshy midrib, and at the base are developed two pointed lobes which clasp the stalk. The leaf is soft and leathery and always seems succulent, because it retains its moisture; it has a ruffled edge near its base, which gives it room for turning without tearing its margin. Each leaf tips over sidewise toward the stem, and as far as necessary to bring one edge uppermost. Thus the sun cannot reach its upper surface to pump water from its tissues. The ruffled margin of the upper edge is pulled out straight when the leaf stands in this position, while the lower margin is more ruffled than ever. Thus, it stands triumphantly, turning edgewise to the sun, retaining its moisture and thriving when cultivated plants are dry and dying.
A common compass plant.
Photo by Cyrus Crosby.
It also has another "anchor to the windward." A plant so full of juice would prove attractive food for cattle when pastures are dry. The leaves of this perhaps escape, because each has a row of very sharp spines on the lower side of the midrib. At first we might wonder why they are thus placed; but if we watch a grazing animal, as a cow, reach out her tongue to pull the herbage into her mouth, we see that these spines are placed where they will do the most efficient work. The teasel has the same clever way of warning off meddlesome tongues. The prickly lettuce also has spines on its stem, and the leaves are toothed with spines at their points.
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.
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.