The _petiole_ or _midrib may act as a tendril_, as in various kinds of clematis. In Fig. 172, the common wild clematis or “old man vine,” this mode is seen. [Illustration: FIG. 171.--LEAVES OF PEA,--very large stipules, opposite leaflets, and leaflets represented by tendrils.] =Twiners.=--The entire plant or shoot may wind about a support. Such a plant is a twiner. Examples are bean, hop, morning-glory, moonflower, false bittersweet or waxwork (_Celastrus_), some honeysuckles, wistaria, Dutchman’s pipe, dodder. The free tip of the twining branch _sweeps about in curves_, much as the tendril does, until it finds support or becomes old and rigid. Each kind of plant usually coils _in only one direction_. Most plants coil against the sun, or from the observer’s left across his front to his right as he faces the plant. Examples are bean, morning-glory. The hop twines from the observer’s right to his left, or with the sun. [Illustration: FIG. 172.--CLEMATIS CLIMBING BY LEAF-TENDRIL.] SUGGESTIONS.--=136.= Set the pupil to watch the behavior of any plant that has tendrils at different stages of maturity. A vigorous cucumber plant is one of the best. Just beyond the point of a young straight tendril set a stake to compare the position of it. Note whether the tendril changes position from hour to hour or day to day. =137.= Is the tip of the tendril perfectly straight? Why? Set a small stake at the end of a strong straight tendril, so the tendril will just reach it. Watch, and make drawing. =138.= If a tendril does not find a support, what does it do? =139.= To test the movement of a free tendril, draw an ink line lengthwise of it, and note whether the line remains always on the concave side or the convex side. =140.= Name the tendril-bearing plants that you know. =141.= Make similar observations and experiments on the tips of twining stems. =142.= What twining plants do you know, and which way do they twine? =143.= How does any plant that you know get up in the world? =144.= Does the stem of a climbing plant contain more or less substance (weight) than an erect self-supporting stem of the same height? Explain. CHAPTER XVIII THE FLOWER--ITS PARTS AND FORMS The function of the flower is to _produce seed_. It is probable that all its varied forms and colors contribute to this supreme end. These forms and colors please the human fancy and add to the joy of living, but the flower exists for the good of the plant, not for the good of man. The parts of the flower are of two general kinds--those that are directly concerned in the _production of seeds_, and those that act as _covering and protecting organs_. The former parts are known as the =essential organs=; the latter as the =floral envelopes=.