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 

 If.fjf m^fffij' 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. 

 Many plants, of very diverse 

 families, secure a better expo- 

 sure of the leaves to light by 

 climbing. The principal types 

 of climbing plants are four in 



like the common climbing roaag ; peoL. 

 like the poison ivy rfig. 56) and the English ivy 



FIG. 57. The wild frost grape, a 

 typical tendril climber 



This vine is climbing on an American 

 elm. The height up to the branch I 

 is about 69 feet, and the average cir- 

 cumference of the main vine, 3 feet. 

 Redrawn and simplified from a pho- 

 tograph by Robert Ridgway 



number : 



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

 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. 



