74 



INTRODUCTION TO BOTANY 



narrow at the base and wider at the tip, as in the evening prim- 

 rose, so that nearly all the space between the leaves is filled in. 

 During which year of the life of a biennial is its principal 

 work photosynthetic? During which year is it reproductive? 

 Why are beets, carrots, parsnips, 

 and salsify used as food at the 

 end of the first season's growth ? 



72. Leaf mosaics. Any combi- 

 nation of leaves (whether found 

 in rosette plants or on longer 

 stems) in which the space is very 

 fully occupied, with few spaces 

 between the leaves, is called a 

 leaf mosaic. Walls covered with 

 Japanese ivy furnish beautiful 

 examples of leaf mosaics on a 

 large scale, and many of our 

 common house plants illustrate 

 the same phenomenon. In any 

 leaf mosaic many of the leaves 

 occupy a very different posi- 

 tion from that which they would 

 have taken if borne on a vertical 

 stem. 



73. Climbing into the light. 



This vine is climbing on an American Many p l an t s , o f very diverse 



elm. The height up to the branch I if. 



is about 60 feet, and the average cir- families, Secure a better expo- 



cumference of the main vine, 3 feet. sure o f t h e leaves to light by 



Redrawn and simplified from a pho- , . . rr , 1 . . , 



tograph by Robert Ridgway climbing. I he pruicipal types 



of climbing plants are four in 



number : scramblers, like the common climbing roses ; root 

 climbers, like the poison ivy (fig. 56) and the English ivy 

 (fig. 40) ; twiners, like the morning-glory; and tendril climbers, 

 like the grapevine. The only way in which one can get a 

 thorough knowledge of the behavior of climbers is to watch 

 them throughout as much as possible of the growing season. 



FIG. 57. The wild frost grape, a 

 typical tendril climber 



