AN INTRODUCTION TO PLANT BIOLOGY 81 



(Optional.) Cut transverse sections of the pod in several places, 

 and identify the layers in the wall of the pod, the seed-stalk, placenta, 

 dorsal and ventral sutures. Sketch and label. 



Break a pod transversely in several places, and notice strong 

 fibers. On which edge of the pod ? Do you see any relation between 

 the position of the strongest fibers and the natural curvature of the 

 pod ? Why are the green pods called "string-beans " by gardeners ? 



(D) Cut a fresh bean branch having young pods, and place 

 cut end in red ink. After a time note where the pods are colored 

 and draw your conclusions as to the functions of the fibers in the 

 pods. Is there any reason why these fibers should be more abundant 

 on one edge of the pod? 



Look at specimens of pods which have matured, dried, and split 

 naturally. Does the splitting (dehiscence) usually occur on the 

 dorsal or ventral edge, or both? 



Take out a bean and note its markings where it was attached to 

 the seed-stalk. The scar left when the seed-stalk is pulled off is 

 called the hilum. At one side of the hilum (toward the stalk of the 

 pod) is a small translucent elevation with a slit-like marking. On the 

 opposite side of the hilum is a very minute pit known as the mi- 

 cropyle. When pollen-grains touch the stigma, as previously de- 

 scribed in 75, the very small tube that grows from each pollen- 

 grain extends down the style and along the placenta of the ovary 

 to the micropyle of an ovule. Later, when fertilization is com- 

 pleted as described in 75, each ovule develops into a seed, and the 

 entire ovary into the fruit or bean pod. Look at an opened bean pod 

 with the seeds in position, and note whether the micropyle is above 

 the seed-stalk or below (toward the style). Now, make a diagram 

 of a pod with seeds, and by a broken or colored line show the path 

 a pollen-tube must take from the style to an ovule. 



The Bean Seed and its Germination 



78. Varieties of Beans. (L) Compare color, markings, and size 

 of specimens of some of the common varieties of beans grown in 

 gardens. (The school-museum should have a collection of the most 

 common varieties of beans arranged in small labeled bottles or boxes.) 



The variations in color, size, etc., of the seeds are no 

 greater than the variations of all parts of the plants which 

 grow from them : (1) Bean plants may be low (dwarf beans), 

 or climbing (e.g., lima beans) j (2) they may have flowers of 



