AN INTRODUCTION TO PLANT BIOLOGY 73 



often break off the terminal buds of climbing or pole beans 

 (e.g., limas) when they have reached a height of about six 

 feet. This has the effect of sending into the developing 

 fruit (pods) much of the nutriment which would have gone 

 to make the useless extension of the stem. 



(L) Examine buds on a young bean plant, (1) with reference to 

 their position, and (2) separate carefully the parts of both a leaf- 

 and a flower-bud. 



The Bean Leaves 



72. Leaf and Leaflets of Bean Plant. (L) Brief examination 

 of these will show that what we at first take to be the individual 

 leaves are arranged in groups of three (Fig. 28). A study of 

 the development of the bean has 

 led botanists to the conclusion 

 that the three leaves are really 

 three parts or divisions of one 

 leaf. Such a leaf is called com- 

 pound, and the three divisions 

 are called leaflets. The support- 

 ing leaf-stalk (petiole) is attached 

 at a node of the stem and can 

 be easily distinguished from a 

 branch of the stem, because the 



leaf -stalk has an enlarged and nig ht. (From Detmer.) 

 somewhat flexible attachment to 

 the stem; and also because a longitudinal cut through the node 

 shows that the leaf is attached chiefly to the rind, while the branch 

 is attached firmly to the wood of the stem. Each of the leaflets is 

 attached by a thickened joint similar to that of the petiole at the 

 stem. These flexible joints of the leaf and leaflets allow the droop- 

 ing of the leaves at night and on hot days. 



Examine any one of the leaflets, and note that the expanded 

 surface (blade) is a thin sheet of tissues supported by ribs and veins. 

 The central rib is the mid-rib. Hold the leaflet up so that light 

 may shine through it, and with a hand-lens examine the delicate 

 veins (veinlets) between the ribs. Make an outline sketch showing 

 the structure of a complete leaf and the detailed structure of one of 

 the three leaflets ; and label all the parts named above. 



