AN INTRODUCTION TO PLANT BIOLOGY 83 



(dark brown in "yellow six-weeks," light brown in ' ' golden-eyed wax " 

 beans). These conspicuous marks will aid in locating the micropyle 

 on the opposite side of the hilum. Make outline drawings of a bean, 

 looking down upon the hilum, and also as seen from the side. 



Strip off the seed-coat from a water-soaked bean. The seed-coat 

 has two layers, which are easily seen in a green bean. The main 

 body of the bean consists of two thickened halves (seed-leaves or 

 cotyledons). These are stored with food for the early use of the 

 young plant that will grow from the seed. 



Carefully separate the two cotyledons, and notice a pointed rod- 

 like body which is joined to the cotyledons. This is called hypocotyl 

 (Fig. 33, h). It is best seen in a bean which has begun to sprout, for 

 the hypocotyl then pushes through the seed-coat at the micropyle. 

 The part of the hypocotyl next the cotyledons will form the begin- 

 ning of the stem, while the pointed end will form the first root. Before 

 sprouting begins, it is difficult to see any line between the stem and 

 the root part of a hypocotyl ; and we simply call the entire structure 

 hypocotyl until growth makes it easy to distinguish between stem 

 and root. By placing the hypocotyl in certain dyes (e.g., solution 

 of permanganate of potash) the root part quickly takes up the stain 

 and makes it easy to see that the pointed end (root) of the hypocotyl 

 is different in its cells from the upper or stem part. 



The words caulicle and radicle which are used in some botanical 

 books are practically synonymous with hypocotyl. 



Joined to the hypocotyl where this is united with the cotyledons 

 is a very short stem with a pair of small leaves. Between these 

 leaves is a small bud. This short stem with leaves and bud consti- 

 tutes the epicotyl. It will form the stem and leaves above the 

 cotyledons. Some books call the bud with the small leaves a plumule, 

 but for the beginning of the stem and leaves above the cotyledons 

 the word epicotyl is preferred. 



80. The Bean Embryo. Cotyledons, hypocotyl, and 

 epicotyl together constitute the embryo, which may de- 

 velop into a bean plant. The part of the plant which de- 

 velops from each part of the embryo is as follows : 



Cotyledons not very useful as working leaves 

 of bean seedling, but stored with food. 



Bean embryo 

 consists of 



Hypocotyl forms stem below cotyledons and 

 root at its lower end. 



Epicotyl forms stem and leaves above coty- 

 ledons. 



