324 A TEXT-BOOK OF BOTANY. 



A number of plants ascend into the air on other plants, or 

 other objects which serve as supports, either by attaching them- 

 selves to them or by twining around them, when they are dis- 

 tinguished as twiners and climbers. TWINERS ascend by a special 

 circumnutating movement of the stem, as in the morning glory, 

 Menispermum (Fig. 180), etc. CLIMBERS, however, ascend by 

 means of special structures, as the aerial roots of the ivy (root 

 climbers) ; or they may climb by means of leaves, as in Clematis 

 (leaf climbers) ; still others climb by means of tendrils, as in the 

 grape and Bryonia (tendril climbers) (Fig. 181) ; and again 

 plants may climb by means of hooked hairs or spines, as in Rubus, 



FIG. 182. Rhizome of Podophyllum representing three years' growth: b a ,the terminal 

 bud of last year; b 2 , the corresponding one of the present year; B.the terminal one of the 

 entire rhizome will develop in the spring of next year. L 1 and L 2 indicate the scars of aerial 

 leaves of the two preceding years' growth; b l and b 2 , latent buds. After Holm. 



Rosa, etc. The tendrils, which are thread-like modifications of the 

 stem, are in some cases provided with disk-like atachments for 

 holding the plant in position, as in the Virginia creeper. Twiners 

 and climbers are sometimes spoken of as LIANES (lianas), particu- 

 larly those of tropical regions, where they form a prominent 

 feature of the forest vegetation. The lianes usually have rope- 

 like, woody stems, the formation of leaves being either suppressed 

 or retarded, and they often run for long distances over the ground 

 and climb to the tops of the tallest trees. They are also frequently 

 characterized by an anomalous stem-structure, the tracheae being 

 very large. 



Stems vary, furthermore, in size and form. While most stems 

 are more or less cylindrical or terete, other forms also occur, as 



