Fig. 119.—The Common Pitcher Plant (Sarracenia purpurea) showing the tubular leaves and the odd, long-stalked flowers.
Carnivorous Plants.—Certain plants capture insects and other very small animals and utilize them to some extent as food. Such are the sundew, which has on the leaves sticky hairs that close over the insect; the Venus’s fly-trap of the Southern States, in which the halves of the leaves close over the prey like the jaws of a steel trap; and the various kinds of pitcher plants that collect insects and other organic matter in deep, water-filled, flask-like leaf pouches (Fig. [119]).
The sundew and the Venus’s fly-trap are sensitive to contact. Other plants are sensitive to the touch without being insectivorous. The common cultivated sensitive plant is an example. This is readily grown from seeds (sold by seedsmen) in a warm place. Related wild plants in the south are sensitive. The utility of this sensitiveness is not understood.
Parts that Simulate Leaves.—We have learned that leaves are endlessly modified to suit the conditions in which the plant is placed. The most marked modifications are in adaptation to light. On the other hand, other organs often perform the functions of leaves. Green shoots function as leaves. These shoots may look like leaves, in which case they are called cladophylla. The foliage of common asparagus is made up of fine branches: the real morphological leaves are the minute dry functionless scales at the bases of these branchlets. (What reason is there for calling them leaves?) The broad “leaves” of the florist’s smilax are cladophylla. Where are the leaves on this plant? In most of the cacti, the entire plant body performs the functions of leaves until the parts become cork-bound.
Leaves are sometimes modified to perform other functions than the vital processes: they may be tendrils, as the terminal leaflets of pea and sweet pea; or spines, as in barberry. Not all spines and thorns, however, represent modified leaves: some of them (as of hawthorns, osage orange, honey locust) are branches.
Suggestions.—To test for chlorophyll. 84. Purchase about a gill of wood alcohol. Secure a leaf of geranium, clover, or other plant that has been exposed to sunlight for a few hours, and, after dipping it for a minute in boiling water, put it in a white cup with sufficient alcohol to cover. Place the cup in a shallow pan of hot water on the stove where it is not hot enough for the alcohol to take fire. After a time the chlorophyll is dissolved by the alcohol which has become an intense green. Save this leaf for the starch experiment (Exercise 85). Without chlorophyll, the plant cannot appropriate the carbon dioxide of the air. Starch and photosynthesis. 85. Starch is present in the green leaves which have been exposed to sunlight; but in the dark no starch can be formed from carbon dioxide. Apply iodine to the leaf from which the chlorophyll was dissolved in the previous experiment. Note that the leaf is coloured purplish-brown throughout. The leaf contains starch.86. Secure a leaf from a plant which has been in the dark for about two days. Dissolve the chlorophyll as before, and attempt to stain this leaf with iodine. No purplish-brown colour is produced. This shows that the starch manufactured in the leaf may be entirely removed during darkness.
| Fig. 120.—Excluding Light and CO2 from Part of a Leaf. | Fig. 121.—The Result. |
87. Secure a plant which has been kept in darkness for twenty-four hours or more. Split a small cork and pin the two halves on opposite sides of one of the leaves, as shown in Fig. [120]. Place the plant in the sunlight again. After a morning of bright sunshine dissolve the chlorophyll in this leaf with alcohol; then stain the leaf with the iodine. Notice that the leaf is stained deeply except where the cork was; there sunlight and carbon dioxide were excluded, Fig. [121]. There is no starch in the covered area. 88. Plants or parts of plants that have developed no chlorophyll can form no starch. Secure a variegated leaf of coleus, ribbon grass, geranium, or of any plant showing both white and green areas. On a day of bright sunshine, test one of these leaves by the alcohol and iodine method for the presence of starch. Observe that the parts devoid of green colour have formed no starch. However, after starch has once been formed in the leaves, it may be changed into soluble substances and removed, to be again converted into starch in certain other parts of the living tissues. To test the giving off of oxygen by day.