The sense of touch is a less obvious one than either vision or hearing. There is no specialized visible organ of touch such as the eye and the ear; it is a generalized sense with individual cells scattered all over the skin. You realize how important it is to you when your foot falls asleep and you have no idea whether it is touching the floor or not.
WRITING ASSIGNMENT: Again build up a basic vocabulary of touch as you have done with the other senses. Have a cardboard box with two holes on opposite sides large enough to admit a pair of hands. In this box have an object which has good tactile qualities such as a small branch, a shell or a stuffed toy. Let a student feel the object with both hands and describe it to the class. See if they can guess what it is from his adjectives of touch. It’s a good game which should elicit many tactile words, which they can record in notebooks.
Response of Sensitive Plant to touch
We have discussed only humans and animals so far; plants should get their turn. Sensitive plant is well known for its ability to move when touched. It makes a good lab subject.
It is easily grown from seeds which you can purchase from the Connecticut Valley Biological Supply Co. Inc., PO Box 326, Southampton MA, 01073. Seeds are also available at local nurseries. It will take a few months before the plants are large enough to be used, though. We usually have several plants growing at the McCabe Center which you are welcome to borrow. (call 787-8758).
ACTIVITY: Tell your students that they are going to test the sensitive plant’s responses to touch. They should be as observant as possible and write down all their lab notes in the notebook. Have them work in groups of four or five, each with a magnifying glass, (available at the Teachers Institute). They must be very careful not to jar the plants or the leaves will fold up. (A wonderful book to show them is
Mimosa, The Sensitive Plant
, by Selsam and Wexler. It has marvellous large color photos. See Student Bibliography.)
First have them draw the plant in their notebooks. Each compound leaf is made of several sprays with leaflets. If they look carefully at base of each leaf they will see a tiny swelling called a
. At the base of each leaf stalk there is a larger pulvinus.
Touch a leaflet pulvinus gently with a needle. What happens? (The pair of leaflets fold up immediately, and all the other leaflets on that spray may close up too if the impulse is strong enough. Now touch the large pulvinus at the base of the leaf stalk and the whole compound leaf will move down. If you touch it hard enough all the leaves will collapse. Why? The pulvinus is a water filled swelling which when touched loses some of its water and no longer props up the leaflet or leaf.
Finally bang the whole pot on the table and everything will collapse. Have a SENSITIVE PLANT CONFERENCE bringing everyone’s observations together. Ask for guesses as to why sensitive plant moves when touched. (By the way, its scientific name is
means mimic and shy.)
At the end of lab have your students plant their own seeds.